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WISDOM EDUCATION…

Force & Laws of Motion

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Force & Laws of Motion

What Causes Motion?

(a) An external force is required to make a stationary body move. (b) Similarly, an external force is required to stop a moving body.

Balanced and Unbalanced forces:

(a) Balance forces are:

Equal and opposite forces and,

Do not causes any changes in the state of motion. Example - Tug of war.

(b) Unbalanced forces are:

Unequal forces and,

Can be in the same or opposite direction and,

Will definitely causes a change in motion.

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Laws of Motion: the first law of motion is given by Aristotle i.e.,

(1) Aristotle’s Law of Uniform Motion: according to him, there is always an external force is required to keep a body in uniform motion.

But later on Aristotle’s law was discarded because, an external force is required to keep a body in uniform motion, only if resistive forces (like frictional & viscous forces) are present. If the resistive forces are not present then the always be in motion irrespective of the external force.

(2) Galileo’s Law: A body moving on a frictionless surface (smooth surface) should move with constant velocity. Aristotle’s law was falsified by Galileo.

Newton’s First law of motion: (Law of Inertia) according to him, A body at rest tends to remain at rest and a body in uniform motion tends

to remain in the state of uniform motion until & unless an external force is applied on it.

OR A body at rest tends to remain at rest and a body in uniform motion tends

to remain in the state of uniform motion until & unless an unbalanced force acts on it.

What is Inertia?

Inertia is the resistance of a body to change its state of motion. For e.g. (a) Jerk experienced when brakes of a vehicle is applied suddenly. (b) Tendency to bend on one side on a bike during sharp turn.

Inertia can be of two types; inertia of rest and inertia of motion. We can better understand this with the help of an example.

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Newton’s second law: As we have studied that, Newton’s 1st law was for the scenario’s where the

net force = 0.

But, now we study that, Newton’s 2nd law is for the scenarios where the net force ≠ 0, which means there is some force acting on the body.

What is Momentum (p)?

Whenever a body is motion i.e. have some velocity it have some momentum, & that Momentum is product of mass of body and its velocity.

It is a Vector quantity.

It is denoted by p=mv.

Dependency of force:

Greater force is required to set heavier bodies in motion.

Greater force is required to stop bodies moving with higher speed.

Therefore, greater the change in momentum in a given time, greater is the force that need to be applied. Therefore according to above conclusions: F ∝ m and F ∝ v Hence, F ∝ mv

F ∝ p

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Thus According to Newton’s second law of motion, The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. F ∝ (dp/dt) (where, dp/dt is rate of change of momentum) F = k (dp/dt) Alternatively, The relationship between an object’s mass m, acceleration a, and applied force F is F = ma, the direction of force and acceleration is same. Derivation of F = ma:

Unit of Force =

Consistency of Second Law with First Law:

According to 2nd law: F = ma If, F = 0 a = 0, i.e. If acceleration of a body is zero, it implies that the body is either at rest or uniform motion, and

According to 1st law: if a= 0 Fnet = 0

Impulse: is defined as a force multiplied by time it acts over. OR In other word if a considerable amount of force acts on a body for a very short period of time, it is said to be impulse. Example: Tennis racket striking the ball.

According to definition, Impulse = F x t = (p x t)/t = p Therefore, Impulse = change in momentum

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Newton’s Third Law of Motion:

To every action, there is always an equal and opposite reaction. Action and Reaction Forces:

Action & Reaction forces always act on different bodies.

Action & Reaction forces occur at the same instant.

Conservation of momentum: (very very important)

In an isolated system, the total momentum is conserved. (An isolated system is a system in which no external force acts on a body).

Example: (will be discussed in detail in classroom program)

A bullet fired from a rifle:

Therefore ……………………………………………………………………………………………………

(a) Rocket propulsion:

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Collision of two bodies:

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Very Short answer type questions:

(1) What name is given to the product of mass and velocity of a given body? (2) Name the physical quantity which is considered to be the measure of the

quantity of motion of a body? (3) What is SI unit of momentum? (4) State whether momentum is scalar or vector? (5) What is the total momentum of a bullet and a gun before firing? (6) Name the physical quantity whose unit is kgms-1? (7) What is the momentum of a body is mass is m and velocity is v? (8) What is the usual name of forces which cannot produce motion but

changes the shape of the body? (9) Name the unbalanced force which slows down the moving bicycle when

we stop paddling it? (10) Unbalanced forces acting on a body changes its shape. (T/F) (11) When a ball is dropped from a height, its speed increases gradually, name

the force that causes this. (12) Name the property of bodies to resist the change in their state of rest or

motion. (13) What is the other name of newton’s 1at law of motion? (14) Mass of A is 6kg and that of B is 34kg, which one of A and B has more

inertia? (15) Name the scientist who gave the laws of motion. (16) State whether the force is scalar or vector quantity. (17) With which physical quantity should the speed of the bull be multiplied to

get its momentum? (18) .................................................................is the measure of inertia of a body. (19) When a running car stops suddenly, the passengers are jerked..................... (20) When a stationary car starts suddenly, the passengers are jerked................. (21) Newton’s 1st law of motion is also called Galileo’s law of……………………......... (22) If there were no unbalanced force of……………….......................................and

no…………………….......resistance, a moving bicycle will go on moving forever. (23) Calculate the momentum of a toy of mass 200g and velocity 5m/s.

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Short answer type questions:

(1) Explain why, it is easier to stop a tennis ball than s cricket ball moving with a same speed.

(2) Explain the meaning of following equation, p=mv. (3) Explain how a karate player can break a pile of tiles with a single blow of

his hand. (4) A body of mass 25 kg has a momentum of 125kgms-1. Calculate the velocity

of the body. (5) What is the change in momentum of a care weighing 1500kg when its

speed changes from 36km/h to 72km/h uniformly? (6) Calculate the momentum of following:

(a) An elephant of mass 2000kg and velocity 5m/s. (b) A bullet of mass 0.02kg moving with velocity 400m/s.

(7) Which of the two, balanced or unbalanced forces can change the shape of the body? Give example.

(8) Describe the term Inertia with respect to motion. (9) State with example to illustrate the Newton’s 1st law of motion.

(10) On what factor the inertia of a body depends? Which one have more inertia, a cricket ball or a rubber ball of same size?

(11) Why the passengers tend to fall backward when the bus starts suddenly? (12) Why the passenger fall forward when the bus stops suddenly? (13) Give reason: When the hanging carpet is beaten with a stick, the dust

particles starts to come outside. (14) When a tree is shaken, its fruits and leaves fall down. Why? (15) Explain why it is dangerous to jump out of a moving bus. (16) What is the momentum in kgms-1 of a 10kg car travelling at:

(a) 5m/s (b) 20 cm/s, and (c) 36 m/s?

Long answer type questions:

(1) Define momentum of a body. On what factors does it depends? (2) Calculate the change in the momentum of a body of 5kg when the speed of

it changes from 20m/s to 0.20m/s. (3) Define the term force. State the various effects of force. (4) Why are road accidents at high speeds very much worse than those at low

speed?

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(5) Give one example each where: (a) A force moves a stationary body. (b) A force stops a moving body. (c) A force changes the speed of a moving body. (d) A force changes the direction of the moving body. (e) A force changes the shape and size of the body.

(6) What type of forces – balanced or unbalanced – act on a rubber ball when we press it in our hands? What effect is produced in the ball?

(7) What happens to the passenger of a bus when it take a sharp turn? Give reason for your answer.

Multiple Choice Questions:

(1) When a toothpaste is squeezed, its shape changes. The force responsible for this is an example of: (a) Balanced forces (b) Centripetal forces (b) Unbalanced forces (d) Centrifugal forces

(2) The inertia of an object tends to cause an object: (a) To increase its speed. (b) To decrease its speed. (c) To resist the change in its state of motion. (d) To decelerate due to friction.

(3) When we talk of a force acting on a body, it usually means: (a) Electrical force (c) Balanced force (b) Unbalanced force (d) Nuclear force

(4) A passenger in a moving train passes a coin which falls behind him, this shows the motion of train is: (a) Accelerated (b) Uniform (c) Retarded (d) Along circular track

(5) ‘When the hanging carpet is beaten with a stick, the dust particles starts to come out of it’. This can be explained best by Newton’s: (a) 3rd law of motion (c) Law of gravitation (b) 1st law of motion (d) 2nd law of motion

(6) A water tanker filled up to two third of its tank with water is running with uniform speed. When the brakes are applied suddenly, the water in its tank will be, (a) Move backward (b) Move forward (c) Rise upward (d) Remain unaffected

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(7) The inertia of a moving object depends on its: (a) Momentum (b) Speed (c) Mass (d) Shape

(8) If we release a magnet held in our hand, it falls down. The force responsible for this is: (a) Balanced force (c) Unbalanced force (b) Magnetic force (d) Muscular force

(9) When a rubber balloon is pressed between hands, its shape changes. This happens because, (a) Balanced force act on the balloon (b) Unbalanced force act on the balloon (c) Frictional forces act on the balloon (d) Gravitational forces act on the balloon

(10) Which of the following effect cannot be produced by the unbalanced forces acting on the body? (a) Change in speed of body (c) Change in direction of motion of body (b) Change in shape of body (d) Change in state of rest of body

High Order Thinking (HOT’s):

(1) A plastic ball and a clay ball of equal masses, travelling in same direction with equal speeds, strike against a vertical wall. From which ball does the wall receives the greater amount of momentum?

(2) A bicycle stops after some time if we stop paddling, but Newton’s 1st law states that, a body in motion remains moving unless an external force is applied on it. How would you explain this case?

(3) A man throws a ball 500g vertically upward with a speed of 10m/s. (a) What will be the initial momentum? (b) What will be the momentum at the highest point of the flight?

(4) A car is moving on a level road. If the driver turn off the engine the speed of car gradually decreases and stops. A student says, two forces act on the car which bring it to stop. What could these forces be? Which of these contribute more to slow down and stop the car?

(5) These are two types of X and Y. The forces belonging to type X can produce motion in a stationary object but cannot change shape of the object, while Y cannot produce motion but change the shape of object. What is the general name of the forces such as (a)X & (b)Y?

(Space for solving these questions is given at the back)

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Very Short Answer Questions:

(1) Which physical quantity corresponds to the rate of change of momentum? (2) State the relation between the momentum of a body and force acting on it. (3) What is the unit of force? (4) Define 1 newton force. (5) What is the relationship between forces and acceleration? (6) If the mass of body and the force acting upon it both are doubled, what will

happen with the acceleration? (7) Name the physical quantity whose unit is Newton. (8) Which physical principle is involved in the working of a jet aeroplane? (9) Name the principle on which rocket works.

(10) State true/false: A rocket can propel itself in vacuum. (11) What is the force which produce 1m/s2 acceleration in a body of 1kg

mass? (12) Find the acceleration produced by a 5N force acting on a body of 10kg. (13) A girl weighing 25kg standing on the floor exerts 250N on the floor, what

force does the floor acts on the girl? (14) Name the physical quantity which makes it easier to accelerate a small car

than a large car. (15) To every action, there is an…………….…….......and............................. reaction. (16) Momentum is ...............................quantity, its unit is………………………............ (17) Newton’s 2nd law state that Force = mass x ................................................or

Force = ..............................of change of…………………………………………............... (18) Forces in Newton’s 3rd law pair have equal...........but act in opposite……...... (19) In collisions and explosions, the total ................................remains constant,

provided that no external........................................................................acts.

Short Answer Questions:

(1) Explain the meaning of the following equation: F = ma Where the symbols have their usual meanings.

(2) To take a boat away from bank, the boatman pushes the bank with an oar. Why?

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(3) Explain how a rocket works. (4) Why does the gunman get a jerk on firing a bullet? (5) If action is always equal to reaction, explain why a cart pulled by a horse

move? (6) Do action and reaction act on the same body, or different bodies? How are

they related to the magnitude and direction? Are they simultaneous or not?

(7) If a man jumps out of a boat, the boat moves backward. Why? (8) Why is it difficult to walk on slippery road? (9) Explain why, the runner presses the ground with his feet when he start his

run? (10) A 60g bullet fired from a5kg gun leaves with a speed of 200m/s. Find the

speed with which the gun recoils. (11) A 10g bullet travelling at speed of 200m/s strikes and remains embedded

in a target of 5kg which is initially at rest and free to move. By what speed will the target move?

(12) A body at rest of mass 2kg.What force will require to move it with 30m/s after 1s.

(13) A body of mass 5kg moving with a speed of 10m/s, a force is applied which makes it move in 25 sec with 35m/s. Find the value of the force.

(14) A car of mass 2400kg moving with a velocity of 20m/s is stopped after 10seconds on application of brakes. Calculate the retardation and retarding force.

(15) For how long the force of 100N act on a body of 20kg so that it acquires a speed of 100m/s?

(16) How long will it take a force of 10N to stop a mass of 2.5kg moving at 10m/s speed?

(17) The velocity of a body changes from 4m/s to 8m/s when a force is applied for 2s. (a) What is the momentum before force acts? (b) What is the momentum after force acts? (c) What is the gain of momentum per second? (d) What is the value of the force?

(18) A gun of mass 3kg fires a bullet of mass 30g. The bullet takes 0.003s to move through the barrel of gun and acquire a speed of 100m/s. Calculate:(a) the recoiling speed of the gun. (b) Recoiling force on gunman.

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(19) Explain with labelled diagram, how a rocket engine provides a force to move the rocket upward?

(20) Name the laws involved in following situations: (a) The sum of products of masses and velocities of two bodies before and

after collisions remains same. (b) A body of mass 5kg can be easily accelerated than a 50kg body under

same conditions. (c) When a body A is standing on a roller skates pushes another body B

(also on roller skates) and makes him move right side then the A moves left side by same distance.

(d) When there are no friction and no air resistance then the moving bicycle will go on moving forever.

Long Answer Questions:

(1) State and explain Newton’s 2nd law of motion. (2) A 100kg vehicle moving with a speed of 20m/s is brought at rest in distance

of 50m: (a) Find the acceleration (b) calculate the unbalanced force acting on it.

(3) Explain why a cricket player moves his hand backward while catching the ball?

(4) A 150g ball travelling with a speed of 30m/s strikes the palm of a player and stops in 0.05second. Find the force exerted by the ball on the hand.

(5) State Newton’s 3rd law of motion with two example. (6) Explain why when a fire man directs the powerful water stream on fire

from hose pipe, the hose pipe tends to go backward? (7) State the law of conservation of momentum. (8) discuss the conservation of momentum in each case:

(a) a rocket taking of the ground (b) a flying jet aeroplane (9) If a balloon filled with air and its mouth untied is released with the mouth

in downward direction, moves upward. Why? (10) An unloaded truck weighing 2000kg has a maximum acceleration of

0.5m/s2, what is the maximum acceleration when it carries a load of 2000kg?

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Multiple Choice Questions:

(1) A rocket works on the principle of conservation of: (a) Mass (b) Energy (c) Momentum (d) Velocity

(2) An object of 2kg is sliding with a constant velocity of 4m/s on a frictionless horizontal table. The force required to keep the object moving with same velocity is: (a) 32N (b) 0N (c) 2N (d) 8N

(3) The physical quantity that makes it easier to accelerate a small car than the large car is measured in unit: (a) m/s (b) kg (c) kgms-1 (d) kgms-2

(4) According to the 3rd law of motion action and reaction, (a) Act on same body but in opposite direction. (b) Act on different body but in opposite direction. (c) Have same magnitude and direction. (d) Act on either body at normal to each other.

(5) The unit of momentum is: (a) m/s (b) kgms-1 (c) N (d) Kgms-2

(6) A boy of mass 50 kg standing on the ground exert a force of 500N on ground, find the force exerted by ground on the boy: (a) 5N (b) 50N (c) 500N (d) 25000N

(7) Acceleration produced by a motion of 5N acting on a body of mass 20kg in m/s2 : (a) 4 (b) 100 (c) 0.25 (d) 2.5

High Order Thinking (HOT’s):

(1) Why are car seat belts designed to stretch somewhat in a collision? (2) The troops or soldiers equipped to be dropped by parachutes from aircraft

are called paratroopers. Why do paratroopers roll on landing? (3) Why would an aircraft be able to fly on the moon? (4) Explain why it is possible by a small animal to fall from a small height

without injury when it reaches the ground? (5) A girl of mass 50 kg jumped out of a rowing boat of 300kgon the bank with

horizontal velocity 3m/s. With what velocity will the boat begin to move backward?

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(6) A body of 50kg running at 5m/s jumps to a 20kg trolley travelling in same direction with 1.5m/s. What is their common velocity?

(7) A truck of mass 500kg moving with a speed of 4m/s collides another truck of 1500kg moving in same direction with 2m/s. What is their common velocity after collision?

(8) A ball A with mass 1kg moving with 2m/s has a head-on-collision with an identical ball Y at rest. Calculate the velocity of Y if X stops after collision and Y moves off.

(9) A heavy car 2000kg travelling with 10m/s had a head on collision with a sport car B 500kg. If both stops dead after collision, what was the velocity of car B.

(10) A man wearing a bullet proof vest stands still on roller skates. The total mass is 80kg. A bullet of 20g is fired at400m/s. It is stopped by the vest and falls on the ground. What is then the velocity of the man?

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