To move a stationary object or to stop a moving object, there is an effort required. That effort is Force. A force can be used to change the magnitude of velocity of an object (that is, to make the object move faster or slower) or to change its direction of motion. Force can change the shape and size of objects. Some examples where force is used are when a drawer is pulled, lifting a child, kicking a ball, moving a trolley etc.
Balanced and Unbalanced forces:
Balanced force: When two individual forces are equal in magnitude and are opposite in direction, they are known as balanced force. They do not change the state of rest or state of motion of the object.
Unbalanced force: When two individual forces have unequal magnitudes and are opposite in direction, they are known as unbalanced force. The object moves in the direction of the greater force. An unbalanced force brings an object in the motion.
Friction: Friction is a force between two surfaces in contact. It acts in the opposite direction of the moving object.
When we stop pedaling, the bicycle slows down because of the friction force acting in the opposite direction of the motion.
An object moves with a uniform velocity when the forces (pushing force and frictional force) acting on the object are balanced and there is no net external force on it.
If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion. Thus, to accelerate the motion of an object, an unbalanced force is required. And the change in its speed (or in the direction of motion) would continue as long as this unbalanced force is applied. However, if this force is removed completely, the object would continue to move with the velocity it has acquired till then.
Newton’s First Law of Motion
The first law of motion states, “An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.”
The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. It is also known as law of inertia.
A real-life example of law of inertia is travelling in a motorcar. We are in the state of rest while the car is moving . When brake is applied, the car slows down, and we tend to be at rest with respect to the seat. This is due to inertia. The body will remain at rest until an external unbalanced force is applied on it.
Inertia and Mass:
Inertia: It is the state of object wherein the object stays in motion or at rest until an external force is exerted.
Heavier or more massive objects offer larger inertia. The inertia of an object is measured by its mass.
Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
The mass of an object is a measure of its inertia.
Second law of motion:
The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force. The momentum, p of an object is defined as the product of its mass, m and velocity, v.
That is, p = mv
Momentum has both direction and magnitude. Its direction is the same as that of velocity, v. The SI unit of momentum is kilogram-metre per second (kg m s-1).
MATHEMATICAL FORMULATION OF SECOND LAW OF MOTION:
Suppose an object of mass, m is moving along a straight line with an initial velocity, u. It is uniformly accelerated to velocity, v in time, t by the application of a constant force, F throughout the time, t. The initial and final momentum of the object will be, p1 = mu and p2 = mv respectively.
The change in velocity p2-p1
mv-mu
∝ m (v-u)
The rate of change of momentum
The applied force
F
F=
F=kma
K is a proportionality constant and it is chosen to be one.
F=ma
The unit of force is kg m s-2 or newton, which has the symbol N.
Third Law of Motion:
The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
These two forces are always equal in magnitude but opposite in direction.
It also states that to every action there is an equal and opposite reaction.
A real life example of this law is walking. While walking, we push the road below backwards. The road exerts an equal and opposite reaction force on our feet to move forward.
Conservation of momentum:
Law of conservation of momentum states that the sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them. The total momentum of the two objects is unchanged or conserved by the collision.
Let two objects A and B of masses mA and mB travel in the same direction at velocities uA and uB respectively. Let uA>uB and the two balls collide with each other. During collision which lasts for time t, the ball A exerts a force FAB on ball B and the ball B exerts a force FBA on ball A. Let vA and vB be the velocities of A and B after collision.
The momentum of ball A before collision= mA uA
The momentum of ball A after collision= mA vA
The rate of change of momentum of A during the collision is
Similarly, the rate of change of momentum of B during the collision is
According to third law of motion, FAB = – FBA
= –
mAuA + mBuB = mAvA + mBvB
This is law of conservation of momentum.
When an object is dropped from a height, it falls towards the earth. The planets go around the sun, the moon goes around the earth. Gravitational force is acting upon all these objects.
Gravitation:
Centripetal force: The force acting on a body moving in a circular path and is directed towards the centre around which the body is moving. The motion of the moon around the earth is due to the centripetal force. The centripetal force is provided by the force of attraction of the earth.
Gravitational force: The force of attraction between objects with masses is known as gravitational force. The force between sun and the planets is gravitational force.
Universal Law of Gravitation:
The universal law of gravitation states that every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centres of two objects.
Let A and B be two objects of masses M and m respectively. Let d be the distance between these two objects and F be the force of attraction.
According to law of gravitation, the force between two objects is directly proportional to the product of their masses
And the force between them is inversely proportional to the square of the distance between them
G is called universal gravitation constant. Its SI unit is Nm2kg-2. The value of G is 6.67×10-11 Nm2kg-2.
The universal law of gravitation explains several phenomena like force that binds us to earth, the motion of the planets around the sun, motion of the moon around the earth, the tides due to the moon and the sun.
Free Fall:
An object is said to be in free fall when it falls towards the earth under gravitational force.
Acceleration due to gravity: When an object falls towards the earth, there is an acceleration due to the earth’s gravitational force. This acceleration is called acceleration due to gravity. It is denoted by g. Its unit is m/s2.
According to Newton’s second law of motion, F=ma=mg
From universal law of gravitation,
F= G Mm⁄d²
mg= G Mm⁄d²
g= G M⁄d²
If the object is on or near the surface of the earth, then d will be the radius of the earth
Therefore,
g= G M ⁄ R²
The value of g: Substituting the values of universal gravitational constant G=6.7×10-11Nm2kg-2, mass of the earth M=6×1024kg, and radius of the earth R=6.4×106m in g= G M ⁄ R² we get g=9.8m/s2
MOTION OF OBJECTS UNDER THE INFLUENCE OF GRAVITATIONAL FORCE OF THE EARTH
The acceleration due to gravity experienced by an object is independent of its mass. Therefore, all objects fall at same rate. In equations of motion, a can be replaced by g. a is positive when it is in the direction of motion and negative when it opposes the motion.
v= u + at
s= ut + ½ at²
2as= v² – u²
Mass: Mass of an object is the measurement of its inertia. The mass of an object is constant and does not change from place to place.
Weight: Weight of an object is the force with which it is attracted towards the earth. The force of attraction of the earth on an object is known as the weight of the object. It is denoted by W.
W = mg
The SI unit of weight is newton(N). The weight is a force acting vertically downwards, it has both magnitude and direction. The weight of an object is different in different locations.
Weight of the object on the moon:
The weight of the object on the moon is the force with which the moon attracts that object. The mass of the moon is less than the mass of the earth, therefore, moon exerts lesser force of attraction on objects.
Wm = G Mm/R² (Mass of moon=7.35×1022 and radius of moon=1.74×106 )
Wε = G Mm/R² (Mass of earth=5.98×1024 and radius of earth=6.37×106)
Wm/Wε = 1/6
Weight of the object on the moon= 1/6×Weight of the object on the earth
Thrust: The force acting on an object perpendicular to the surface is called thrust. The effect of thrust depends on the area on which it acts.
Pressure: The thrust on unit area is called pressure.
pressure= thrust/area
Its unit is N/m2. Its SI unit is Pascal denoted as Pa.
Pressure in fluids:
All liquids and gases are fluids. Fluids have weight and they exert pressure on the base and walls of the container in which they are enclosed. The pressure is transmitted undiminished in all directions.
Buoyancy:
The upward force exerted by the water is known as upthrust or buoyant force. When an object is immersed in water, it experiences an upwards force due to which the object is pushed upwards. This upward force is greater than the object’s weight. To keep the object completely immersed, the upward force on the object due to water must be balanced. This can be achieved by an externally applied force acting downwards. This force must at least be equal to the difference between the upward force and the weight of the object. Examples of buoyancy are feeling light after swimming, ship made of iron and steel doesn’t sink, but the same amount of iron and steel would sink.
WHY OBJECTS FLOAT OR SINK WHEN PLACED ON THE SURFACE OF WATER?
The objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid. For example, cork floats and iron nail sinks.
Archimedes’ Principle:
Let us consider an activity to understand this topic. Take a stone and tie it with a rubber string or spring balance. Suspend it in the air and note the elongation in the string or the reading of the spring balance. Now, dip it into water and note the reading.
We observe that the elongation decreases when dipped into water and once it is fully immersed, the elongation is same. We know that elongation is caused due to the weight of the stone. But when dipped in water, does the weight of the stone change? No.
There is an upward force which is acting on the stone in the upward direction which decreases the net force acting upon the stone, resulting in the decrease of elongation. This upward force is called Buoyant Force. This buoyant force is different for different fluids. According to Archimedes’ principle, “When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it”. That’s why, the elongation is different in different mediums.
Where is this principle applicable in real life? It plays a main role in designing of ships and submarines. It is also used in lactometer which determines the purity of a sample of milk and hydrometer which determines the density of liquids.
Relative Density:
Density of a substance is defined as mass of a unit volume. The unit of density is kg/m3. It is different for different substances. For example, the density of gold is 19300 kg/m3 while that of water is 1000 kg/m3. The density of a given sample of a substance can help us to determine its purity. The relative density of a substance is the ratio of its density to that of water:
Relative Density = Density of a substance/ Density of water
It has no unit since it’s a ratio.
Motion is everywhere in our surroundings. Motion is the change in position of an object with respect to time. It can either be direct or indirect.
For example, we see a car moving, that is directly indicating that the car is in motion. The person sitting inside the car is in rest according to them, but for us the person is in motion. This indicates an indirect evidence of motion.
Objects show different kinds of motion. Some objects move in straight path while few objects move in circular path. Few objects vibrate while others rotate.
Describing motion:
The location of an object is described using a reference point. The distance of the object is measured from the distance between the object and the reference point. The reference point is taken as origin.
Motion along a straight line:
It is the simplest type of motion. The distance is the total measurement of the movement of an object. Displacement is the shortest distance between the initial and the final position of the object. Distance and displacement are used to describe the overall motion of an object and to locate its final position with reference to its initial position at a given point.
Uniform motion and Non-Uniform motion:
Uniform motion: When the object covers equal distances in equal intervals of time, it is said to be in uniform motion. Ex: When a car cover 50kms in first hour, 50kms in second hour and 50kms in third hour.
Non-uniform motion: When the object covers unequal distances in equal intervals of time, it is said to be in non-uniform motion. Ex: When a car covers 50kms in first hour, 40kms in second hour and 60kms in third hour.
Measuring the rate of motion:
Speed: The rate of motion which is measured by finding out the distance travelled by the object in a unit time. The SI unit of speed is metre per second. It is represented as m/s or ms-1.
When the object is in non-uniform motion, the average speed is calculated.
Average speed: It is obtained by dividing the total distance travelled by the total time taken.
average speed = Total distance travelled/ Total time taken
If an object travels a distance s in time t then its speed v is,
v = s/t
Speed with direction:
Velocity: It is the speed of an object moving in a definite direction. The velocity can be uniform or variable.
If the velocity is changing at a uniform rate, then average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time.
average velocity = (initial velocity + final velocity) / 2
vav = (u + v) / 2
where vav is the average velocity, u is the initial velocity and v is the final velocity of the object.
Rate of change of velocity:
In uniform motion, the velocity remains constant, so the change in velocity for any time interval is zero. In non-uniform motion, the velocity keeps changing with time. It has different values at different instances of time and the change in velocity is calculated by acceleration.
Acceleration: It is the measure of change in velocity of an object per unit time.
acceleration = change in velocity / time taken
If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration a is,
a = (v – u) / t
This kind of motion is known as accelerated motion. The acceleration is taken to be positive if it is in the direction of velocity and negative when it is opposite to the direction of velocity. The SI unit of acceleration is ms-2.
Uniform acceleration: If an object moves in straight line and the change in velocity is equal in equal instance of time, it is said to be in uniform acceleration. Ex: Motion of a freely falling body.
Non-uniform acceleration: If an object changes its velocity in equal instances of time, it is said to be in non-uniform acceleration. Ex: When a car is changing its speed in equal instance of time.
Graphical representation of motion:
Distance-time graph:
It is the representation of change in the position of an object with time. x-axis: time and y-axis: distance
In uniform motion, the distance travelled by the object is directly proportional to time taken, thus the graph is a straight line.
The speed of the object can be found out by
v = (s2 – s1) / (t2 – t1)
In non-uniform motion, the nature of the graph is non-linear.
Velocity-time graphs:
It is the representation of change in velocity of an object with time. x-axis: time and y-axis: velocity
At uniform velocity, the slope will be parallel to x-axis.
The area enclosed by velocity-time graph and the time axis will be equal to the magnitude of the displacement. The distance s moved by the car in time (t2 – t1) can be expressed as
s = area of the rectangle ABCD
For uniformly accelerated motion, the velocity-time graph is a straight line.
s = AB × BC + ½ (AD×DE)
For non-uniform accelerated motion, the velocity-time graph can have any shape.
Equations of Motion by Graphical Method:
There are 3 equations of motion:
v = u + at
s = ut + ½ at²
2as = v² – u²
EQUATION FOR VELOCITY-TIME RELATION:
The initial velocity of the object is u (at point A) and then it increases to v (at point B) in time t. The velocity changes at a uniform rate a.
The initial velocity(u) is represented by OA, final velocity(v) by BC, time(t) is represented by OC and acceleration is a.
From the graph,
BC = BD + CD
BC = BD + OA
v = BD + u
BD = v-u 
The acceleration of the object
a = change in velocity / time taken =BD/AD = BD/OC = BD/t
BD = at
v – u = at
v = u + at
EQUATION FOR POSITION-TIME RELATION:
Let us consider that the object has travelled a distance ‘s’ in time ‘t’ under uniform acceleration ‘a’. The distance travelled by the object is obtained by the area enclosed within OABC under the velocity-time graph AB.
Thus, the distance s travelled by the object is given by
s = area OABC
s = area of the reactangle OADC + area of the triangle ABD
s =OA × OC + ½ (AD × BD)
Substituting OA = u, OC= AD= t and BD = at, we get
s = u × t + ½(t × at)
s = ut + ½ at²
EQUATION FOR POSITION–VELOCITY RELATION:
The distance s travelled by the object in time t, moving under uniform acceleration a is given by the area enclosed within the trapezium OABC under the graph. That is,
s = area of the trapezium OABC = {(OA + BC) ×OC}/2
Substituting OA = u, BC = v and OC = t, we get s = (u+v) / t
t = (v-u) / t (from velocity – time relation)
s= {(v+u) (v-u)}/ 2a = (v² – u²) / 2a
Uniform Circular Motion:
When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.
The velocity v of an object moving in a uniform circular motion of radius r in t seconds is,
v = 2Πr / t
Some of the examples of uniform circular motion are motion of the moon and the earth, a satellite in a circular orbit around the earth, a cyclist on a circular track at constant speed.
Sound:
Sound is a form of energy which produces a sensation of hearing in our ears. Sounds like bells, machines, birds, humans, televisions etc.
Production of sound:
Sound can be produced in different ways by plucking, scratching, shaking or blowing different objects. Vibrating objects produce sound. Vibration is a rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords. Birds flapping its wings produces sound, vibrating rubber band produces sound.
Propagation of sound:
Sound is produced by vibrating objects. The sound transmits through a substance or matter which is known as the medium. It can be solid, liquid or gas. Sound travels from point of generation to the listener through the medium. When an object vibrates, it sets the particles of the medium in vibration. The vibrating object sets the particle adjacent to it in motion and comes back to its original position. This way the disturbance keeps moving in the medium and reaches the listener. The disturbance that moves through a medium which sets neighboring particles into motion is wave. In propagation of sound, the disturbance is carried forward, hence they are visualized as waves. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.
Air is the most common medium for sound propagation. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C). This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).
As the objects moves back and forth rapidly, a series of compression and rarefaction is created in the air which makes the sound wave. Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.
Sound needs a medium to travel:
Sound is a mechanical wave and it needs a medium for its propagation. It can travel in mediums like air, water, steel etc.
Sound can’t travel in vacuum. It is proved by bell jar experiment.
As shown in figure, an electric bell is inserted in a bell jar and is connected to a vacuum pump. When you press the switch, you will be able to hear the bell. When vacuum pump is started and the air is pumped out, the sound becomes fainter and when almost all the air is pumped out, you’ll be able to hear very feeble sound.
Sound waves are longitudinal waves:
Longitudinal waves are the waves in which the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. Sound waves are longitudinal waves because the particles oscillate back and forth about their position of rest. They propagate in the medium as a series of compressions and rarefactions.
There exists another type of wave known as transverse wave. Transverse wave are the waves in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. In transverse wave particles oscillate up and down about their mean position as the wave travels. Light is a transverse wave.
Characteristics of a sound wave:
As the sound wave propagates, the density and pressure vary with distance. (a) shows the density variation of the sound wave and (b) shows the pressure variation of the sound wave.
Compressions are thicker regions and are represented by upper portion of the curve. These are the regions where density and pressure are high. Rarefactions are thinner regions and are represented by lower portion of the curve. These are the regions where density and pressure are low. Peak represents the region of maximum compression, it is known as crest and valley represents the region of maximum rarefaction which is known as trough.
Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is called wavelength. It is denoted by λ. The SI unit is meter.
Oscillation: The change in density from the maximum value to the minimum value, again to the maximum value makes one complete oscillation.
Frequency: The number of oscillations per unit time is known as frequency. It is denoted by ν. The SI unit is hertz.
Time period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. It is the time taken for one complete oscillation in the density of the medium. It is denoted by T. The SI unit is second.
Frequency and time period are related as, v = 1 / T
Pitch: The characteristic of sound based on the frequency of an emitted sound. The faster the vibration of the source, the higher is the frequency and the higher is the pitch. A high pitch sound corresponds to a greater number of compressions and rarefactions passing a fixed point per unit time.
Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is denoted by A. The unit can be of density or pressure. Amplitude determines the loudness or softness of a sound. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. Loud sound can travel a larger distance as it is associated with higher energy.
A sound of single frequency is called a tone. The sound which is produced due to a mixture of several frequencies is called a note.
Speed of sound: The distance travelled by a point on a wave per unit time is called the speed.
speed v = distance / time
v = δ / t
v = δv (v = 1/t)
Intensity of sound: The amount of sound energy passing each second through unit area is called the intensity of sound.
Speed of sound in different media:
Sound propagates through a medium at a finite speed. Speed of sound is less than the speed of light. The speed of sound depends on the properties of the medium through which it travels. The speed of sound in a medium also depends on temperature and pressure of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature, the speed of sound increases.
Reflection of sound:
Sound reflects in the same way as light does. It also gets reflected at the surface of a solid or liquid and follows the same laws of reflection. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface, and the three are in the same plane.
Echo:
If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. The sensation of sound persists in our brain for 0.1 seconds. To hear an echo the time interval between the original sound and the reflected one must be at least 0.1s. The total distance covered by the sound from the point of generation to the reflecting surface and back should be at least 34.4 m. For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m. Distance changes with temperature of air. Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.
Reverberation:
The repeated reflection that results in this persistence of sound is called reverberation. It mostly happens in big halls and auditorium. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fiberboard, rough plaster, or draperies. The seat materials are also selected on the basis of their sound absorbing properties.
Uses of multiple reflection of sound:
- Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience.
- Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.
- Generally, the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall. Sometimes a curved soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall.
Range of hearing:
The audible range for human beings lies between 20Hz to 20000Hz. Children under the age of five and few animals can hear up to 25000Hz. As people age, their ability to hear decreases.
Infrasound: Sound of frequencies below 20Hz is called infrasonic sound or infrasound. Rhinoceros communicate using infrasound. Whales and elephants produce sound in the infrasound range. Earthquakes produce low-frequency infrasound before the main shock wave begins which can be heard by few animals.
Ultrasound: Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by dolphins, bats and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture.
Applications of Ultrasound:
Ultrasounds are high frequency waves. They are able to travel along well-defined paths even in the presence of obstacles. They are used extensively in industries and for medical purposes.
- Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tube, odd shaped parts, electronic components etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects thus get thoroughly cleaned.
- Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are generally used in construction of big structures like buildings, bridges, machines and also scientific equipment. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect.
- Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echocardiography’.
- Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumors in different organs. In this technique the ultrasonic waves travel through the tissues of the body and get reflected from a region where there is a change of tissue density. These waves are then converted into electrical signals that are used to generate images of the organ. These images are then displayed on a monitor or printed on a film. This technique is called ‘ultrasonography’. Ultrasonography is also used for examination of the foetus during pregnancy to detect congenial defects and growth abnormalities.
- Ultrasound may be employed to break small ‘stones’ formed in the kidneys into fine grains. These grains later get flushed out with urine.
SONAR:
SONAR- Sound Navigation and Ranging. It is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. Sonar consists of a transmitter and a detector and is installed in a boat or a ship. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are then interpreted. The distance at which the object is present can be found out if we know the speed of sound in water and the time interval between transmission and reception of the ultrasound. The distance is calculated by 2d = v × t. This method is called echo-ranging. The sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc.
Structure of human ear:
Ear is a sensitive device due to which we are able to hear. It allows us to convert pressure variations in air with audible frequencies into electric signals that travel to the brain via the auditory nerve.
The outer ear is called pinna. It collects the sound from surroundings. The sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane.
When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and the eardrum moves inward. When rarefaction reaches it, the eardrum moves outward. This way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. These amplified pressure variations are transmitted to inner ear. In the inner ear, they are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.
Lightning:
We see little sparks of electricity when a plug is loose in its socket. Lightning is an electric spark at a huge scale. Lightning is caused by the accumulation of charges in the clouds.
When we take off woolen or polyester clothes in the dark, we see a spark and hear crackling sound. In 1752 Benjamin Franklin, an American scientist, showed that lightning and the spark from your clothes are essentially the same phenomena.
Charging by rubbing:
When a plastic comb is rubbed with dry hair, it acquires a small charge. The plastic comb becomes a charged object and hair also gets charged. Therefore, objects can be charged by rubbing
Types of charges and their interaction:
Suppose two objects are charged by rubbing, they acquire two kinds of charge. One object acquires positive charge and the other object acquires negative charge. Like charges repel each other and unlike charges attract each other. For example, when a glass rod is rubbed with a silk cloth, it acquires a positive charge. A plastic straw is also charged by rubbing it with a polythene. When the glass rod is brought near the plastic straw, they are attracted to each other. As they are attracted, the charge acquired by plastic straw is negative. The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current.
Transfer of charge:
An electroscope is a device used to test whether an object is carrying a charge or not. A simple electroscope can be made with an empty jar bottle. Place a cardboard on top of the jar and pierce it and insert a paper clip as shown in the figure. Pieces of aluminium foil are hung on the paper clip. A charged refill is touched with paper clip.
The aluminium foil strips receive the same charge from the charged refill through the paper clip (metals are good conductors). The strips carrying similar charges repel each other and they become wide open. Thus, we find that electrical charge can be transferred from a charged object to another through a metal conductor. When we touch the paper clip with our hand the foil strip collapse. The foil strips lose charge to the earth through our body. We say that the foil strips are discharged. The process of transferring of charge from a charged object to the earth is called earthing.
The story of lightning:
During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause separation of charges. The positive charges accumulate on the upper edges of cloud and the negative charges accumulate near the lower edges. There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning. The process is called an electric discharge. The process of electric discharge can occur between two or more clouds, or between clouds and the earth.
Lightning safety:
During lightning and thunderstorm no open place is safe. We should rush to a safe place when we hear thunder. We should wait for some time before coming out to an open place when we hear the last thunder. A house or a building is a safe place. If you are travelling by car or by bus, you are safe inside with windows and doors of the vehicle shut.
If we’re outside, then open vehicles like bike, tractors are not safe. We should stay away from tall trees, poles and metal objects. Do not lie on the ground. Instead, squat low on the ground. Place your hands on your knees with your head between the hands. This position will make you the smallest target to be struck.
Inside the house, telephone cords, electric wires and metal pipes should be avoided. Wired phones should also be avoided. Bathing should be avoided during thunderstorms to avoid contact with running water. Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.
Buildings are protected from lightning by using lightning conductor. It is a metallic rod which is taller than the building installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides easy route for the transfer of electric charge to the ground. The metal columns used during construction, electrical wires and water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.
Earthquakes:
Natural phenomena like cyclone, thunderstorm, lightning can be predicted. The weather department can warn about a thunderstorm developing in some area. If a thunderstorm occurs, there is always a possibility of lightning and cyclones accompanying it. So, we get time to take measures to protect ourselves from the damage caused by these phenomena.
But an earthquake cannot be predicted, and it can cause damage to human life and property on a huge scale. A major earthquake occurred in India on 8th October 2005 in Uri and Tangdhar towns of North Kashmir. Before that, a major earthquake occurred on 26th January 2001 in Bhuj District of Gujarat.
An earthquake is a sudden shaking or trembling of the earth lasting for a very short time. It is caused by a disturbance deep inside the earth’s crust. Major earthquakes are less frequent but cause immense damage to buildings, bridges, dams and people. Earthquakes can cause floods, landslides, and tsunamis. A major tsunami occurred in the Indian Ocean on 26th December 2004. All the coastal areas around the ocean suffered huge losses.
Tremors are caused by the disturbances deep down inside the uppermost layer of the earth, called the crust. The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision, they cause disturbance in the earth’s crust. It is this disturbance that shows up as an earthquake on the surface of the earth. Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth, or an underground nuclear explosion is carried out.
Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plane. Some areas of South India also fall in the danger zone.
The power of an earthquake is expressed in terms of magnitude on a scale called Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale. Both Bhuj and Kashmir earthquakes had magnitudes greater than 7.5.
Protection against earthquakes:
In highly seismic zones, the buildings should be quake safe. The use of mud or timber is better than the heavy construction material and roofs should be kept as light as possible. The cupboards and shelves should be fixed to the walls. Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have firefighting equipment in working order.
At home:
- Take shelter under a table and stay there till shaking stops.
- Stay away from tall and heavy objects that may fall on you.
- If you are in bed, do not get up. Protect your head with a pillow.
Outdoors:
- Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground.
- If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.
Sound
Sound plays a very important part in our life. It helps us to communicate with one another. We hear various kinds of sounds in our surroundings. There are musical instruments which produce sound, animals produce sound, tress produce sound etc.
Sound is produced by a vibrating body:
The to and fro or back and forth motion of an object is known as vibration. Vibrating objects produce sound. In some cases, we can see the vibration. In most of the cases, we can’t see the vibrations because the amplitude is very small. When we pluck a tightly stretched band, it vibrates and produces sound. When it stops vibrating, it does not produce any sound. The most common example of vibration which produces sound is musical instruments like Veena and Guitar.
Sound produced by humans:
In humans, sound is produced by the voice box or the larynx. Voice box is present at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
The vocal cords in men are about 20mm long. In women these are about 5mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.
Sound needs a medium for propagation:
Sound needs a medium to travel. It can travel through solid, liquid and gases. Sound can travel through wood, iron, water, air etc. Sound can’t travel in vacuum.
We hear sound through our ears:
The shape of the outer part of the ear is like a funnel. When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly. It is called the eardrum. The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.
Amplitude, Time period and Frequency of a vibration:
Oscillatory motion: The to and fro motion of an object is called oscillatory motion.
Frequency: The number of oscillations per second is called frequency of oscillation. It is expressed in hertz and is denoted by Hz.
The sounds are differentiated by their amplitude and frequency.
Loudness and Pitch:
The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.
The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is high, we say that the sound is shrill and has a higher pitch. If the frequency of vibration is lower, we say that the sound has a lower pitch. A bird makes a high-pitched sound whereas a lion makes a low-pitched roar. However, the roar of a lion is very loud. Usually the voice of a woman has a higher frequency and is shriller than that of a man.
Audible and Inaudible sounds:
A human ear can’t detect a sound below 20Hz and above 20kHz. The range of audible frequencies is from 20Hz to 20kHz.
Noise and Music:
The unpleasant sound is called noise. The sounds produced by horns of trucks or buses, sound from constructing sites etc. Musical sound is one which is pleasing to ear. The sound coming from musical instruments are melodious and pleasing to hear.
Noise Pollution:
Presence of excessive or unwanted sounds in the environment is called noise pollution. Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers etc. Lack of sleep, hypertension (high blood pressure), anxiety and many more health disorders may be caused by noise pollution. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.
Measures to limit noise pollution:
To the sources of noise pollution, silencing devices must be installed in aircraft engines, transport vehicles, industrial machines, and home appliances. In residential area, the noisy operations must be conducted away from any residential area. Noise producing industries should be set up away from such areas. Use of automobile horns should be minimised. TV and music systems should be run at low volumes. Trees must be planted along the roads and around buildings to cut down on the sounds reaching the residents, thus reducing the harmful effects of noise pollution.
WORK:
Its definition is different in scientific terms and in day-to-day life.
Work done in day-to-day life: There are a lot of activities we do in our everyday lives which we call work. Watching a movie, singing a song, studying hard for exams, all these are considered as work when we talk about it in general. But it is not considered as work scientifically. If we try to push a big rock, we get exhausted after sometime. It is not considered as work unless the rock is moved.
Scientific conception of work: Scientifically, work is done when force is acting on an object and the object is displaced. If any of the criteria does not exist, then no work is done. Some of the examples are pushing a table and the table moves, pulling a trolley, lifting a book through a height.
Work done by a constant force:
Let F be the force acting on an object, s be the displacement of the object in the direction of force and W be the work done. Work is defined as the product of the force and displacement.
work done = Force × displacement
W = Fs
Work: Work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. Work has only magnitude and no direction. The unit of work is newton meter (Nm) or joule (J).
If the force exerted is in the direction of displacement, then the work done is positive. If the force exerted is in the opposite direction of the displacement, then the work done is negative.
Energy:
An object having a capability to do work is said to possess energy. The object which does the work loses energy and the object on which the work is done gains energy. An object that possess energy can exert force on another object, the energy is transferred to the other object and thus the other object may do work. The energy possessed by an object is thus measured in terms of its capacity of doing work. Therefore, the unit of energy is also joules (J). In our daily lives, there are many sources of energy like nuclei of atoms, interior of earth, and tides. The biggest natural source of energy is the Sun.
Forms of energy: Energy is provided in many forms. The various forms include potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Kinetic Energy:
The energy possessed by a moving object is known as kinetic energy. The kinetic energy of an object increases with its speed. An object moving faster can do more work than an identical object moving relatively slow. The kinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.
Consider an object of mass m moving with a uniform velocity u. Let it now be displaced through a distance s when a constant force, F acts on it in the direction of its displacement. We know that the work done W=Fs. s can be found out using the equation of motion
2as = v² – u²
s = (v² – u²) / 2
We know that F=ma. F and s can be substituted in W=Fs
W = ma {(v² – u²)/ 2a}
W = ½m ( v² – u²)
If the object is starting from its stationary position, u=0, then
K.E = ½ mv²
Potential Energy:
The potential energy possessed by the object is the energy present in it by virtue of its position or configuration. The energy stored in an object is stored as potential energy if it is not used to cause a change in the velocity or speed of the object.
Potential energy of an object at a height:
Gravitational potential energy is present in an object when it is raised, the energy is increased while raising it because of the work done on the object against the gravity. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
Consider an object of mass, m. Let it be raised through a height, h from the ground. A force is required to do this. The minimum force required to raise the object is equal to the weight of the object, mg. The object gains energy equal to the work done on it. Let the work done on the object against gravity be W.
W = force × displacement = mgh
Therefore,
P . E = mg h
The work done depends on the difference in vertical heights of final and initial positions of the object and not on the path along which the object is moved.
Energy can be converted from one form of energy to another.
Law of conservation of energy:
According to the law of conservation, energy can only be converted from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation remains the same.
For example, when an object of mass ‘m’ is dropped from a height ‘h’, the kinetic energy at the start is zero because there is no velocity and the potential energy is maximum i.e., mgh. As the object is falling down, the potential energy will change into kinetic energy. As the height will be decreasing, the potential energy will be decreasing and as the velocity will be increasing, the kinetic energy will be increasing. When the object will be near the ground, h will be zero and v will be highest. Thus, the kinetic energy will be maximum and potential energy will be zero. The sum of the potential energy and kinetic energy of the object would be the same at all points. That is,
potential energy + kinetic energy = constant
mgh + ½ mv² = constant
The sum of kinetic energy and potential energy of an object is its total mechanical energy.
Rate of doing work:
Power is defined as the rate of doing work or the rate of transfer of energy. It measures the speed of work done, how fast or slow work is done. If an agent does a work W in time t, then power is given by:
Power = Work done / time
P = W / t
The unit of power is watt having the symbol W.
The power varies with time. Therefore, average power is used. Average power is obtained by dividing the total energy consumed by the total time taken.
Commercial unit of energy:
Kilowatt hour(kWh) is used for bigger unit of energy. 1 kW h is the energy used in one hour at the rate of 1000 J/s. 1 kWh=3.6×106J.
The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of ‘units. Here, 1 ‘unit’ means 1 kilowatt hour.
