Physics 9th Chapter 3 Dynamics Exercise Questions

Physics 9th Chapter 3 Dynamics Exercise Questions

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Q 3.2 Define the following terms.

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

(i) Inertia: Inertia is that characteristic of a body due to which it resists any change in its state of rest and or of uniform motion in a straight line. It depends on mass of the body.

(ii) Momentum: The quantity of motion of a body it possess due to its mass and velocity is called momentum.

(iii) Force: Force is the agency which moves or tends to move, stops or tends to stop the motion of an object in a straight line. It formula is F = ma. Its unit is Newton.

(iv) Force of friction: The force that opposes the motion of an object is called force of friction.

(v) Centripetal force: The force that keeps a body in a circle is called centripetal force. Its formula is Fc = mv²/r

Q 3.3 What is the difference between?

(i) Mass and weight.

(ii) Action and reaction.

(iii) Sliding friction and rolling friction.

Ans: (i) Mass and weight:

Mass: Mass of a body is the quantity of matter possessed by the body. It is a scalar quantity. It is represented by m. It does not change with the change of place. Its unit is kg.

Weight: Weight of a body is equal to the force with which Earth attracts the body towards its center. It is a vector quantity. It is represented by W. It varies from point to point depending upon the value of g. Its unit is newton.

(ii) Action and reaction:

Action: The force exerted by the first body on the second body is called action.

Reaction: The force exerted by the second body in the response on the first is called reaction.

(iii) Sliding friction and rolling friction:

Sliding Friction: The force between the sliding object which opposes the relative motion between them is called sliding friction.

Rolling Friction: The force of friction between rolling body and the surface over which it rolls is called rolling friction.

Q 3.4 What is the law of inertia?

Ans: Newton’s first law: Everybody continues its state of rest or of uniform motion in a straight line provided no net force acts upon it. Newton’s first law of motion deals with inertial property of matter so, it is also known as law of inertia.

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

Ans: It is dangerous to travel on the roof of a bus because when the driver applies the brakes, the passengers on the roof may fall due to inertia. Due to inertial property, the passengers on roof of bus want to continue their motion with the bus. They can fall due to inertia.

Q 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.

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

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

Explanation: Consider a body of mass m moving with initial velocity vi . When a force acts upon a body an acceleration is produced in it, this change the velocity vf. If Pi and Pf be the initial momentum and final momentum of the body related to initial and final velocities respectively then.

Pi = mvi

Pf = mvf

Change in Momentum = final momentum – initial momentum

Pf  - Pi = mvf  - mvi

(Pf - Pi)/t= (vf - mvi)/t 

(Pf - Pi)/t= m (vf - vi)/t  ----(I)

Since (vf - vi)/t=a

Δ(Pf - Pi)/t = ma ----(II)

According to Newton’s second law of motion,

F = ma put in eq II

Δ(Pf - Pi)/t = F 

Or F= ΔP/t  This equation shows that rate of change of momentum is equal to applied force.

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

Ans: A rope that is pulled from its ends by two opposite forces 100N each then the tension will be 100N.

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

Ans: Action and reaction are always equal and opposite but body moves because action and reaction are applied on two different bodies. The two forces can only cancel if they act on same object.

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

Ans: A horse pushes the Earth in the backward direction, this is the action and in reaction Earth  pushes the horse in the forward direction. In this way cart moves, as action and reaction are applied on different bodies.

Q 3.11 What is the law of conservation of momentum?

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

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

Ans: Law of conservation of momentum is an important law and has vast applications. It is applicable universally not only for smaller bodies like electrons & protons but also for bodies like planets and stars.

Q 3.13 When a gun is fired, it recoils, why?

Ans: Consider a system of gun and a bullet. Before firing the gun, both 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 is smaller than the velocity of bullet because mass of gun is greater than the bullet.

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

Ans: (i) Friction is needed to walk on the ground.

(ii) It is risky to run on wet floor with shoes that have smooth soles.

(iii) Friction is required by birds to fly in the sky.

(iv) Athletes use special shoes that have extraordinary ground grips. Such shoes prevent them from slipping while running fast.

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

Ans: Because liquid friction is less than solids. No surface is completely smooth. By oiling the surface, the pits and bumps are filled with oil and so friction is reduced. Also surfaces become slippery due to oiling.

Q 3.16 Describe ways to reduce friction?

Ans: The friction can be reduced by following methods:

(i) Making the sliding surfaces smooth.

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

(iii) Lubricating the sliding surfaces e.g. (oil or grease is used for this purpose)

(iv) Using ball bearings or roller bearings. Because rolling friction is lesser than the sliding friction.

Q 3.17 Why rolling friction is less than sliding friction?

Ans: The rolling friction is less that sliding friction because:

(i) The contact between wheel and surface is only at one point.

(ii) There is no relative motion between wheel and surface.

Q 3.18 What you know about the following.

Ans: (i) Tension is a string: The force exerted by a string when it is subjected to pull. Tension and weight are in opposite direction. It is a vector quantity. Its SI unit is newton. It is represented by T.

(ii) Limiting force of friction: The maximum value of friction is known as the force of limiting friction (Fs). It depends on the normal reaction (pressing force) between the two surfaces in contact.

(iii) Braking force: The maximum force of friction between tyres of vehicles and surface of road which is necessary to stop the vehicle is called Braking force.

(iv) Skidding of vehicles: If force of friction between tyres of vehicles and roads is not sufficient and brakes are put on too strongly, the wheels of car will lockup and its directional control will be lost. This is called skidding of vehicles.

(v) Seatbelts: Seatbelts are the belts connected with the seats of vehicles. It stops the upper part of passengers to move forward due to inertia, and save the passengers from serious injuries.

(vi) Banking of road: Banking of road means that the outer edge of a road track is kept higher than the inner edge.

(vii) 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. It is used to separate cream from milk.

Q 3.19 What would happen if all friction suddenly disappear?

Ans: If all the friction suddenly disappears then bodies can’t be moved and can’t be turned and even not change its state. We cannot walk without friction. We would keep sliding. Noting would be steady on the ground. We cannot hold anything. We cannot turn a page and cannot write with pen. Everything will collapse if there will be no friction.

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

Ans: The spinner of a washing machine moves with a very high speed to provide required centrifugal force, due to which undesired particles can be easily removed from the clothes.