Physics Class 9 Chapter 3 (Exercise Solution)

 

UNIT No. 3

Dynamics

Exercise

3.1        Encircle the correct answer from the given choices.

i.               Newton's first law of motion is valid only in the absence of:

(a) force                         (b) net force                (c) friction                   (d) momentum

ii.             Inertia depends upon

(a) force                         (b) net force                (c) mass                       (d) velocity

iii.           A boy jumps out of a moving bus. There is a danger for him to fall:

(a) towards the moving bus

(b) away from the bus

(c) in the direction of motion

(d) opposite to the direction of motion

iv.           A string is stretched by two equal and opposite forces 10 N each. The tension in the string is

(a) zero                          (b) 5 N                       (c) 10 N                       (d) 20 N

v.             The mass of a body:

(a) decreases when accelerated

(b) increases when accelerated

(c) decreases when moving with high velocity

(d) none of the above.

vi.          Two bodies of masses m₁ and m₂ attached to the ends of an inextensible string passing over a frictionless pulling such that both move vertically. The acceleration of the bodies is:

\[(a) \frac{m_{1}m_{2}}{m_{1}+m_{2}}\]

\[(b) \frac{m_{1}- m_{2}}{m_{1}+ m_{2}}\]         

\[(c)\frac{m_{1} + m_{2}}{m_{1}- m_{2}}\]            

\[(d)\frac{2m_{1}m_{2}}{m_{1}+m_{2}}\]

vii.         Which of the following is the unit of momentum?

(a) Nm                           (b) kgms^-2                   (c) Ns                          (d) Ns^-1

viii.       When horse pulls a cart, the action is on the:

(a) cart                           (b) Earth                      © horse                       (d) Earth and cart

ix.           Which of the following material lowers friction when pushed between metal plates?

(a) water                                                            (b) fine marble powder

(c) air                                                                 (d) oil

Answers of Multiple Choice Questions (MCQs)

i.	(b)	ii.	(c)
iii.	(c)	iv.	(c)
v.	()	vi.	()
vii.	(c)	viii.	(c)
ix.	(b)	x.	(d)
xi.	(a)	xii.	(b)

 

3.2        Define the following terms:

(i) Inertia                      (ii) Momentum                      (iii) Force      

(iv) Force of friction    (v) Centripetal force

Ans: Inertia

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

Momentum

“Momentum of a body is the quantity of motion it possesses due to its mass and velocity”

Momentum is a vector quantity. It is represented by P . The momentum  P of a body of mass m moving with velocity v is given by the relation

                                                               P = mv

SI unit of momentum is kgms^-1.

Force

“A force 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”

Force of Friction

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

Centripetal

“Centripetal force is a force that keeps a body to move in a circle.”

The Centripetal force is given by the following formula

\[F_{c} = \frac{mv^{2}}{r}\]                                                                 

3.3        What is the difference between:

(i) Mass and weight

(ii) Action and reaction

(iii) Sliding friction and rolling friction

Ans: Difference between mass and weight

Sr. No.	Mass	Weight
1.	Mass of a body is the quantity of matter possessed by the body.	Weight of a body is equal to the force with which earth attracts it.
2.	It is a scalar quantity.	It is a vector quantity.
3.	Its SI unit is kilogram (kg).	Its SI unit is newton (N).
4.	Its value does not change with place.	Its value changes from place to place.
5.	Mass is measured by comparison with standard masses using a beam balance.	Weight is measured with spring balance.

 

Difference Between Action and Reaction

Sr. No.	Action	Reaction
1	Action is a cause.	Reaction is an effect due to the cause.
2	Action is the act that you perform on something expecting something to happen,	Reaction is the happening of something which we are expecting in the result of action.
3	Action force acts on one body.	Reaction force acts on another body.
4	Action force acts in one direction	Reaction force acts in other direction
5	The force exerted on an object is the action,	The force experienced by the object due to action as a consequence of Newton's third law is the reaction.

 

Difference Between Sliding and Rolling Friction

Sr. #	Sliding Friction	Rolling Friction
1	When an object slides over the surface of another object it is called sliding friction.	When an object rolls over the surface of another object it is called rolling friction.
2	Sliding friction takes place due to interlocking between microscopic surfaces.	Rolling friction takes place due to the deformation of surfaces.
3	The coefficient of sliding friction depends on the texture of the surface and temperature to a certain extent.	The coefficient of rolling friction is dependent on the radius of the rolling object, the depth to which the object can sink, and the toughness of the surface
4	It is greater than rolling friction.	It is lesser than sliding friction.
5	Only a point on the surface remains in contact with the object.	A finite area of the surface remains in contact with the object
6	Sliding friction is given by the relation Fs = µs N	Rolling friction is given by the relation. Fr = µr N

 

3.4        What is the law of Inertia?

Ans: Newtons First law of motion is also known as law of inertia it states that

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

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

 

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

Ans: 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 outwards.

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

“When a force acts on a body, it produces an acceleration in the body and will be equal to the rate of change of momentum of the body.”

It is given by the relation

\[F =  \frac{P_{f}-P_{i}}{t}\]

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

Ans: 10 N

Explanation:

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

Ans: Action and reaction do not act on the same body. Action is applied on one body due to which an equal and opposite reaction is acting on another body. Both of these do not cancel each other due to which the body will move.

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

Ans: The cart moves due to the reaction force of ground.

Explanation:

The horse apply force on cart and ground, in the reaction cart and ground both apply force on the horse. The force of horse on cart and the force of cart on horse both cancel each other. The horse and cart both move due to the reaction force of ground.

3.11   What is the law of conservation of momentum?

Ans: The law of conservation of momentum states that

“The momentum of an isolated system of two or more than two interacting bodies remains constant.”

3.12   Why is the law of conservation of momentum important?

Ans: By using law of conservation of momentum, it is possible to calculate force, velocity, acceleration of a body. Most of elementary particles are discovered by use of this law.

3.13   When a gun is fired, it recoils. Why?

Ans: The gun recoils to conserve the momentum of the system.

Explanation

Consider the 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 moves in the forward direction and gun recoils so that total momentum after firing also become zero.

3.14   Describe two situations in which force of friction is needed.

Ans: Following are the situations in which force of friction is needed

1.      We cannot write if there would be no friction between paper and the pencil.

2.      Friction enables us to walk on the ground.

3.      Birds could not fly, if there is no air resistance. The reaction of pushed air enables the birds to fly.s

3.15   How does oiling the moving parts of a machine lowers friction?

Ans: The oil coats the surfaces of the moving parts and makes them slippery. They slide over each other more easily, so there is less friction. Friction is always parallel to the surface in contact.

3.16   Describe ways to reduce friction.

Ans: Following are the methods of reducing friction

·                Making the sliding surfaces smooth.

·                Making the fast moving objects a streamline shape (fish shape) such as cars, aero planes, etc. This causes the smooth flow of air and thus minimizes air resistance at high speeds.

·                Lubricating the sliding surfaces.

·                Using ball bearings or roller bearings. Because the rolling friction is lesser than the sliding friction

3.17   Why rolling friction is less than sliding friction?

Ans: When an object slides over the surface of another object it is called sliding friction. When an object rolls over the surface of another object it is called rolling friction. The force of friction depends on the area of contact between the two surfaces. Greater the area of contact between two surfaces greater is the force of friction. In case of rolling friction, the area of contact between two surfaces is lesser than in case of sliding friction. Therefore, rolling friction is lesser than the sliding friction.

3.18 What you know about the following:

(i)         Tension in a string

(ii)       Limiting force of friction

(iii)       Braking force

(iv)       Skidding of vehicles

(v)       Seatbelts

(vi)       Banking of roads

(vii)      Cream separator

Ans:       

(i)        Tension in a string (Not included in the smart Syllabus)

(ii)              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 can be increased to certain maximum value. It does not increase beyond this. The maximum value of friction is known as the force of limiting friction (F_s). It is given by the formula

F_s = µR

Here R is the normal reaction and µ is the coefficient of static friction.

 

(iii)            Braking force (Not included in the smart Syllabus)

(iv)             Skidding of vehicles (Not included in the smart Syllabus)

(v)              Seatbelt (Not included in the smart Syllabus)

(vi)             Banking of roads (Not included in the smart Syllabus)

(vii)          Cream separator (Not included in the smart Syllabus)

3.19 What would happen if all friction suddenly disappears?

Ans: Following things will happen If all the friction is suddenly disappear

1.      We would not be able to walk

2.      We would not be able to write with pencil

3.      We would not be able to pick objects.

4.      We would not be able to run vehicles and brakes of the vehicles would not work.

In short everything would be out of order and whole system on the earth would collapse.

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

(Not included in the smart Syllabus)