# Dynamics

## 9th Class Physics Chapter 3 Exercise Short Questions with Answers

Q 3.2 : Define the following terms

(i)

Inertia :

” Inertia of a body is the property due to which it resist any change in its state of rest or motion .”

• Inertia of a body is related to the mass of body .
• Greater is the mass of body , greater is its inertia .

(ii)

Momentum :

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

or

” Momentum of a body is equal to the product of mass and velocity .”

Formula : P = m v

Unit : Ns or kgms-1

(iii)

Force :

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

Symbol : F

Unit : Newton (N)

(iv)

Force of Friction :

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

Examples :

• Applying brakes to stop a moving vehicle .
• Writing on a notebook / board .

(v)

What is differentiate between :

(ii)

(iii)

Q 3.4 : What is the law of inertia ?

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 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 .”

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

If a person travels on the roof of a bus , it would be dangerous .

Reason : Because when a bus takes a sharp turn , passengers fall in outward direction . It is due to inertia that they want to continue their motion in a straight line and thus fall outwards .

Q 3.6 : 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 .

Reason : It is due to inertia that they want to continue their motion in a straight line and thus fall outwards .

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

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

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

The tension in a rope that is pulled from its ends by opposite forces 100 N each will be 100 N.

Q 3.9 : Action and reaction are always equal and opposite then how does body move ?

Action and Reaction force always act on different bodies , so they do not cancel the effect of each other .

→ So , we apply Action force on the earth in backward direction , and earth apply Reaction force in forward direction .

→ This is how we move .

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

The horse applies action force by feet on the road , the reaction is given by the road on horse , due to which horse moves .

• The cart , which is tied with the horse , also moves .

Since , action and reaction never acts on same body , so the cart moves.

Q 3.11 : What is the law of conservation of momentum ?

Law of conservation of Momentum :

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

Examples : Firing a bullet , Release of air from balloon etc .

Mathematical : m1v1 + m2v2 = m1u1 + m2u2

Q 3.12 : Why is the law of conservation of momentum important ?

By using law of conservation of momentum , we can calculate :

• Force
• Velocity
• Acceleration

→ Most of elementary particles are discovered by the use of this law .

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

Reason : When a gun is fired it recoils to conserve momentum .

Explanation :

• When the bullet is fired from the gun , it gives a large velocity to the bullet in the forward direction .
• So , no external force acts on the system , so the momentum of the system (gun + bullet ) must be zero after firing .

→ Thus , gun moves backwards with a momentum equal to the momentum of the bullet .

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

1. We need friction to write on the paper .
2. We need friction to walk on the ground .
3. Birds need friction to fly in the air .
4. Friction is needed to stop moving vehicles .

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

Oiling the moving parts of a machine lowers friction because the oil fills up all the rough spot (cold welds ) and make the surface smooth .

Q 3.16 : Describe ways to reduce friction .

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.
• Lubricating the sliding surfaces .
• Using ball bearings or roller bearings . Because rolling friction is less than sliding friction .

Q 3.17 : Why rolling friction is less than sliding friction ?

Reason :

In rolling friction , the area of contact between two objects is comparatively less than in case of sliding .

Hence , area of contact is less ,, so force of rolling friction is less than sliding friction .

Q 3.18 : What you know about the following :

(i)

Tension in a string :

” The force acting along a string causes tension in the string .”

Example :

When a weight is suspended by a string . Tension produces by a string . It is equal to the weight but opposite in direction .

(ii)

Limiting force of friction :

“The maximum value of friction is known as the Force of limiting friction . “

→ Denoted by Fs .

(iii)

Braking Force :

” The force which is required to overcome the limiting friction is called braking force .”

→ It is basically a measure of braking power of a vehicle .

(iv)

Skidding of vehicles :

” The act of sliding or slipping of a vehicle over a surface , often sideways without revolving is called skidding .”

→ It happen when a force of friction between tyres and road is small , then on applying brakes to moving vehicle, its tyre slide over road .

(v)

Seatbelts:

A belt or strap in an automobile or airplanes that is used for safety .

In case of an accident or sudden stop , seat belts holds the passenger in their seats .

(vi)

The phenomenon of raising outer edge of the curved road above the inner edge is to provide necessary centripetal force to the vehicle to take safer turn on the curve road is called banking of roads .

(vii)

Cream Separator :

A device which is used to separate cream from milk .

Q 3.19 : What would happen if all friction suddenly disappears ?

• If there was no friction , Nothing would steady on ground and nothing would exist in the way they do now .

→Birds cannot fly .

→ We cannot walk on ground .

→ We cannot write on the paper etc.

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

Reason :

Because when it spins at high speed , the water from wet clothes is forced out through these holes due to lack of centripetal force .

9th Class Physics Chapter 2 Exercise Short Questions with Answers

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