# 9th Class Physics: Chapter 3 Dynamics Short Questions Answers

## 9th Class Physics: Chapter 3 Dynamics Short Question Answers

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Define following terms:

(i) Inertia (ii) Momentum (iii) Force (iv) Force of Friction (v) Centripetal force

(i) Inertia: Inertia of a body is its property due to which it resists any change in its state of rest or motion.

(ii) Momentum: Momentum of a body is the quantity of motion possessed by the body. Momentum of a body is equal to the product of its mass and velocity.

(iii) Force: A force is a push or pull. It moves or tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body.

(iv) Force of Friction: A force between the sliding objects which opposes the relative motion between them is called force of friction.

(v) Centripetal force: The force which keeps the body to move in a circular path is called centripetal force. Fc = mv2/r

What is the difference between:

1. Mass and weight
2. Action and reaction
3. Sliding friction and rolling friction

1. Mass and Weight:

Mass:

Mass of a body is the quantity of matter that it possesses.

Mass is a scalar quantity

Mass remains same everywhere in the Earth and does not change with change of place.

Weight:

Weight of the body is equal to the force with which Earth attracts it.

Weight is a vector quantity.

Weight does not remain same everywhere in the Earth and changes with change of place.

2. Action and Reaction:

Action:

It is a force that is exerted by a body let A on the other body B is called action force.

Reaction:

It is a force which is exerted by the second body let B on the first body A is called reaction force.

3. Sliding and Rolling:

Sliding:

A force between the sliding objects which opposes the relative motion between them is called sliding friction.

The magnitude of sliding friction is very large as compare to rolling friction.

Rolling friction:

Rolling friction is the force of friction between a rolling body and the surface over which it rolls.

The magnitude of rolling friction is very small as compare to sliding friction.

What is the law of inertia?

Newton’s first law of motion deals with the inertial property of matter, so Newton’s first law of motion is also known as law if inertia.

Statement: A body continues its state of rest or uniform motion in a straight line provided no net force acts on it.

Why is it dangerous to travel on the roof of a bus?

If a person travels on the roof of a bus, it would be dangerous because when a bus takes a sharp turn, passenger falls in the outward direction. It is due to inertia that they want continue their motion in a straight line and thus fall outward.

Why does a passenger move outward when a bus takes a turn?

When a bus takes a sharp turn, passengers fall in the outward direction. It is due to inertia that they want to continue their motion in a straight line and thus fall outward.

How can you relate a force with the change of momentum of a body?

When a force acts on a body, it produces acceleration in the body and will be equal to the rate of change of momentum of the body. We can write it as; change in momentum = final momentum – initial momentum

Pf – Pi = mvf – mvi

Thus, rate of change in momentum is given by.

Pf – Pi/t = m (vf – vi/t)

As, vf – vi/t = a

Pf – Pi/t = ma. ­­­­____(I)

And Newton’s second law of motion tells us that;

F = ma

By putting the value of F in eq. (I)

Pf – Pi/t = F which is the required relation.

What will be the tension in a rope that is pulled from its ends by two opposite forces 100 N each?

The total tension in the rope will be:

T = 100N

Action and reaction are always equal and opposite. Then how does a body move?

According to Newton’s third law of motion, action and reaction are always equal and opposite in direction. But action and reaction forces always act on different bodies, so they do not cancel the effect of each other, and under this condition of forces the body moves irrespective to this, that action and reaction are equal but opposite in direction.

A horse pulls the cart. If the action and reaction are equal and opposite then how does the cart move?

As a horse pulls the cart, it is the action of the horse on the Earth, according to the Newton’s third law of motion; Earth also applies the equal amount of force as a reaction on the horse in opposite direction. This would be cancelled out, if both the forces act on one body, but these two forces act on two separate bodies so they would not cancel the effect of each other and the cart moves.

What is the law of conservation of momentum?

The momentum of an isolated system of two or more than two interaction bodies before and after the collision remains the same is known as law of conservation of momentum. Let two bodies of masses m1 and m2 moving with velocities v1 and v2 respectively before collision and their velocities after collision become u1 and u2 respectively. According to the statement of law of conservation of momentum.

M1v1 + m2v2 = m1u1 + m2u2

Why is the law of conservation of momentum important?

Law of conservation of momentum is very much important because it is applicable on all objects in the universe either larger or smaller. This law has vast applications. According to this law the momentum of an isolated system of two or more than two interacting bodies remains same.

When a gun is fired, it recoils. Why?

Consider a system of gun and a bullet. Before firing the gun, both the gun and bullet are at rest, so the total momentum of the system is zero. As the gun is fired, bullet shoots out of the gun and acquires momentum. To conserve momentum of the system, the gun recoils. According to the law of conservation of momentum, the total momentum of the gun and the bullet will also be zero after the gun is fired. Therefore the gun recoils.

Describe two situations in which force of friction is needed.

There are many conditions in which friction is desirable; two of them is given below.

• Friction is needed when we write, we cannot write if there would be no friction between the paper and the pencil.
• Friction enables us to walk on the ground; we cannot run on slippery ground. A slippery ground offers very little friction. Hence, any body who tries to run on a slippery ground may meet an accident.

How does oiling the moving parts of a machine lower friction.

By making the sliding surfaces as smooth as possible we can reduce the friction and for this, we apply any lubricant such as oil in the moving parts of machine. It will smooth the surfaces which are sliding and hence reduce the friction between these parts of a machine.

Describe ways to reduce friction.

The friction can be reduced by:

• Making the sliding surfaces as smooth as possible.
• Making the fast moving objects a streamline shape such as cars, trains, and aeroplanes. This causes the smooth flow of air around their bodies and thus minimizes air resistance at high speeds.
• Using lubricants between the sliding surfaces.
• Using ball bearings or roller bearings. Because rolling friction is much less than sliding friction.

Why rolling friction is less than sliding friction?

All surfaces have pits and bumps and when two such surfaces are in contact. Then contact points between the two surfaces from a sort of cold welds. These cold welds resist the surfaces from sliding over each other. More is the area of in contact surfaces more would be the friction. In sliding friction more area is in contact. For example, when the axel of wheel is pushed, the force of friction between the wheel and the ground at the point of contact provides the reaction force. The reaction force acts at the contact points of the wheel in a direction opposite to the applied force. The wheel rolls without rupturing the cold welds. That is why the rolling friction is extremely small than sliding friction.

What you know about the following:

• Tension in a string
• Limiting force of friction
• Breaking force
• Skidding of vehicles
• Seatbelts
• Cream separator

Tension in a string:

Consider a block supported by a string. The upper end of the string is fixed on a stand. Let w be the weight of the block. The block pulls the string downwards by its weight. This causes a tension “T” in the string. The tension T in the string is action upwards at the block. As the block is at rest, therefore, the weight of the tension T in the string must be equal and opposite to the weight w of the body.

Limiting force of friction:

Friction is equal to the applied force that tends to move a body at rest. It increases with the applied force friction has a maximum value of friction is known as the force of limiting friction Fs. It depends upon the normal reaction processing force between the two surfaces in contact. And mathematically.

Fs = µR
Where co-efficient of friction= µ
R = normal reaction
Fs = focfceof limiting friction

Breaking force:

To stop a car quickly, a large force of friction between the tyres and the road is needed. But there is a limit to this force of friction that tyres can provide. If the brakes are applied too strongly, the wheel of the car will lock up and the car will skid due to its large momentum. It will lose its direction control that may result in an accident. In order to reduce the chance of skidding, it is advisable not to apply brakes too hard that lock up their rolling motion especially at high speeds. Force exerted by brakes is called braking force.

Skidding of vehicles:

To stop a car or vehicle quickly a large force of friction between the tyres and the roads is needed. But there is a limit to this force of friction that tyres can provide. If the brake are applied too strongly, the wheels of the car will lock up and the car will skid due to its large momentum. Its directional control will be lost and its would meet an accident. In order to reduce the chance of skidding, it is advisable not to apply brakes too hard that lock up their rolling motion especially at high speed. Moreover it is unsafe to drive a vehicle with worn out tyres.

Seatbelts:

In case of an accident, a person not wearing seat belt will continue moving until stopped suddenly by something before him. This something may be a windscreen, another passenger or back of the seat in the front of him. Seatbelts are useful in two ways.

• They provide an external force to a person wearing seat belt.
• The additional time is required time is required for stretching seat belts. This prolongs the stopping time for momentum to change and reduces the effect of collision.

When a car takes a turn, centripetal force is needed to keep it in its curved track. The friction between the tyres and the road provides the necessary centripetal force. The car would skid if the force friction between the tyres and the road is not sufficient enough particularly when the roads are wet. This problem is solved by banking of curved roads. Banking of a road means that the outer edge of a road is raised. Imagine a vehicle on a curved road. Banking cause a component of vehicle’s weight to provide the necessary centripetal force while taking a turn. Thus banking of roads prevents skidding of vehicle and thus makes the driving safe.

Cream separator:

Most modern plants use a separator to control the fat contents of various products. A separator is a high-speed spinner. It acts on the same principle of centrifuge machines. The bowl spins at very high speed causing the heavier contents of milk to move outward in the bowl pushing the lighter content inward towards the spinning axis. Cream or butterfat is lighter than other components in milk. Therefore, skimmed milk, which is denser than cream is collected at the outer wall of the bowl. The lighter part is pushed towards the centre from where it is collected through a pipe.

What would happen if all friction suddenly disappears?

If there was no friction then we could not walk on the ground. Nothing would be steady on the ground and it would be nearly impossible to keep things still as there would be no force to oppose an object motion. Everything would be slide around would not be stopped. There would be no sound because the waves have to be transferred with air friction, many things would be just sliding and sliding. Infact, nothing would exist in universe, without friction.

Why the spinner of a washing machine is made to spin at a very high speed?

Spinner of a washing machine is made to spin at a very high speed. Because when it spins at high speed, the water from wet clothes is forced out through these holes due to lack of centripetal force.

Define Law of Inertia.

Newton’s first law of motion deal with the inertia property of matter, so Newton’s first law of motion is also known as law of inertia.

Statement: A body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.

Define momentum and write the formula.

Momentum: Momentum of a body is the quantity of motion possessed by the body. Momentum of a body is equal to the product of its mass and velocity.

Formula: The momentum P of a body is given by the product of its mass m and its velocity v.

P = mv

Define inertia.

Inertia: Inertia of a body is its property due to which it resists any change in its state of rest or motion.

Differentiate between force and inertia.

Force: A force is a push or pull. It moves or tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body.

Inertia: Inertia of a body is its property due to which it resists any change in its state of rest or motion.

What is meant by dynamics?

Dynamics: The branch of mechanics that deals with the study of motion of an object and the cause of its motion is called dynamics.

Dive two differences between mass and weight.

Mass:

Mass of a body is the quantity of matter that it possesses.

Mass is a scalar quantity

Weight:

Weight of the body is equal to the force with which Earth attracts it.

Weight is a vector quantity.

What is the relation between force and momentum?

When a force acts on a body, it produces acceleration in the body and will be equal to the rate of change of momentum of the body. We can write it as;

Change in momentum = final momentum – initial momentum

Pf – Pi = mvf – mvi

Thus, rate of change in momentum is given by.

Pf – Pi/t = m (vf – vi/t)

As, vf – vi/t = a

Pf – Pi/t = ma. ­­­­____(I)

And Newton’s second law of motion tells us that;

F = ma

By putting the value of F in eq. (I)

Pf – Pi/t = F which is the required relation.

State Newton’s First Law of Motion.

A body continues its state of rest or uniform motion in a straight line provided no net force acts on it.

Give Law of conservation of momentum.

Law of conservation of momentum: The momentum of an isolated system of two or more than two interaction bodies before and after the collision remains the same is known as law of conservation of momentum. Let two bodies of masses m1 and m2 moving with velocities v1 and v2 respectively before collision and their velocities after collision become u1 and u2 respectively. According to the statement of law of conservation of momentum.

M1v1 + m2v2 = m1u1 + m2u2

Define Newton’s third Law of Motion: To every action there is always an equal but opposite reaction.

Example: Take an air-filled balloon. When the balloon is set free, the sir inside it rushes out and the balloon moves forward.

What will be the tension in a string that is pulled from its ends by two opposite forces 100 N each?

The tension is the string is 100 N.

State Second Law of Motion.

Statement: When a net force acts on a body, it produces acceleration on the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.

Prove F = ma

If a force produces an acceleration “a” in a body of mass “m” then it can be stated mathematically that

a ∝ F_____(1)

a ∝ 1/F____(2)

by combining eq. (1) and (2)

a ∝ F/m

a = K

F = ma

What is Atwood Machine? Give its one use.

Atwood Machine: An atwood machine is an arrangement of two objects of unequal masses. Both the objects are attached to the ends of string. The string passes over a frictionless pulley. This arrangement is sometime used to find the acceleration due to gravity by the following formula.

g = a

A body of mass 5 kg is moving with a velocity of 10 ms-1. Find the force required to stop it in 2 seconds.

m = 5kg

vi = 10 ms-1

vf = 0 ms-1

t = 2s

F = ?

Pi =

vi = 10 ms-1

vf = 0 ms-1

t = 2s

F = ?

Pi = mvi

Pi = 5kg x 10 ms-1

Pi = 50 Ns

Pf = mvf

Pf = 5 kg x 0 ms-1

Pf = 0 Ns

Since F =

F =

F = 25 Ns

Thus 25 N, force is required to stop the body.

The weight of a body is 147N. what will be its mass? Value of g is 10 m/s

Weight = w = 147 N

Gravitational acceleration =g = 10ms-1

To find out:

Mass = ?

Formula:

W = mg

M = w/g

Solution: By putting values, we get:

147/10 = m

m = 14.7 kg

The required mass of the body is 14.7 kg.

Find the acceleration produced by a force of 100N in a mass of 50kg.

Sol. Force = F= 100N

Mass = m = 50kh

To find out: Acceleration = a =?

Formula  F = ma

Solution: by putting values, we get;

100 = (50)(a)

a = 100/50

a = 2 ms-2

The required acceleration produced by a force is 2 ms-2.

Define force and its SI unit.

Force: A force is a push or pull. It moves or tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body.

Unit: In SI, unit of force is Newton N.

Newton: One newton is the force that produces an acceleration of 1ms-2 in a body of mass of 1 kg.

How much force is needed to prevent a body of mass 10 kg from falling?

Data

Mass = m = 10 kg

Gravitational acceleration = g = 10 ms-2

To Find out: Force = F =?

Formula: In this case F = w = mg

Solution: By putting values, we get;

F = (10)(10)

F =100N

The required force to prevent a body of mass 10kg is 100N.

Describe two ways to reduce friction.

The friction can be reduced by:

• Making the sliding surfaces as smooth as possible.
• Making the fast moving objects a streamline shape such as cars, trains, and aeroplanes. This causes the smooth flow of air around their bodies and thus minimizes air resistance at high speeds.

What type of shoes are better for jogging and why?

The rough surface shoe is better for jogging. Because it suffers more friction and prevent from slipping.

What is meant by coefficient of friction? Write its mathematical form.

Co-efficient of friction:

The ratio between the force of limiting friction Fs and normal reaction R is constant. This constant is called the co-efficient of friction and is represented by µ.

Formula: µ = Fs/R

If m is the mass of the block, then for horizontal surface

R = mg

Fs = µmg

Fs/mg = µ

Differentiate between siding friction and rolling friction.

Sliding friction:

A force between the sliding objects which opposes the relative motion between them is called sliding friction.

The magnitude of sliding friction is very large as compare to rolling friction.

Rolling friction:

Rolling friction is the force of friction between a rolling body and the surface over which it rolls.

The magnitude of rolling friction is very small as compare to sliding friction.

It cannot be written if there would be no friction between paper and pencil.

Friction enables us to walk on ground. We cannot run on a slippery ground because it offers very little friction.

Friction is undesirable when moving with high speeds because it opposes the motion and thus limits the speed of moving objects.

Most of our useful energy is lost as heat and sound due to the friction between various moving parts of machines.

What is meant by rolling friction?

Rolling friction:

Rolling friction is the force of friction between a rolling body and the surface over which it rolls.
The magnitude of rolling friction is very small as compare to sliding friction.

What is the reason for slipping on wet ground? Explain.

Friction enables us to walk on wet ground. We cannot run on a slippery ground because it offers very little friction.

Define friction and write equation.

The force that opposes the motion of moving objects is called friction.

i.e. F = µR

How does cream separator work?

A separator is a high-speed spinner. It acts on the same principle of centrifuge machines. The bowl spins at very high speed causing the heavier contents of milk to move outward in the bowl pushing the lighter content inward towards the spinning axis. Cream or butterfat is lighter than other components in milk. Therefore, skimmed milk, which is denser than cream is collected at the outer wall of the bowl. The lighter part is pushed towards the centre from where it is collected through a pipe.

Define and write down equation of centripetal force.

Centripetal force: The force which keeps the body to move in a circular path is called the centripetal force.

Fc = mv2/r

Define Centripetal force and Circular Motion.

Centripetal force: The force which keeps the body to move in a circular path is called centripetal force.

Fc = mv2/r

Circular Motion:

The motion of an object in a circular path is known as circular motion.

Define Centrifugal force.

Centripetal reaction that pulls the string outward is sometimes called the centrifugal force.

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