# Circular Motion–9th Science Notes and Question Bank

CHAPTER-17

Circular Motion

I. For each of the following statements or questions four alternatives are given. Choose the most appropriate alternative:

1. The force responsible for drying of clothes in a washing machine is --------

a) Gravitational force b) centripetal force c) centrifugal force d) electrostatic force

2. Force involved in uniform circular motion is given by -------

a) mv2/r b) mvr c) mr2/v d) mrv2

3. A cyclist moving in a circular track leans towards the centre -------------

a) to gain in momentum b) to gain necessary centrepetal force

c) to gain necessary centrifugal force d) to gain speed

II. Fill in the blanks.

1. At any point in a circular motion the direction of linear velocity of a particle is ----- ( changes )

2. The direction of centripetal force is ------ ( towards the centre )

III. Match the followings.

1. Linear speed of a particle in a uniform circular motion - rotatory motion (3)

2. centripetal force -  constant(1)

3. spinning top - circular motion (4)

4. Electrons revolving round a nucleus - towards the centre (2)

1. Write the differences between revolutionary motion & rotatory motion. Give one example to each.

Circular motion in which body moves in a circular path with change in its position is called revolutionary motion eg: giant wheel.

Circular motion in which body moves in a circular path without change in its position is called rotatory motion eg : merry go round.

2. A body moving in a circular path with constant speed is an accelerated motion. Why?

A body moving in a circular path with constant speed is an accelerated motion because direction of motion is different.

3. Define centripetal force. Write an expression for it.

In a circular motion, the radial force acting on the body directed towards the centre is called centripetal force.

Expression for centripetal force = mv2/r, Here m =mass, v = velocity, r = radius

4. Cyclist in circular path leans towards the centre, give reason.

Cyclist in circular path leans towards the centre to gain necessary centripetal force

5. Give illustration for centripetal force.

i. Electrons revolving round a nucleus

ii. A cyclist moving in a circular track leans towards the center

iii. Motion of planets around sun.

iv. Motion of moon around earth.

v. Banking of roads and railway tracks

vi. A runner while taking a sharp bend leans inwards to obtain the necessary centripetal force. If the roadbed is tilted properly, the inclination provides the centripetal force.

vii. A cyclist going round a curve leans towards the centre of the curve in order to get necessary centripetal force.

6. What is centrifugal force?

The pseudo force which try to pull the body outwards in circular motion are called centrifugal force.

7. Give two applications of centrifugal force.

a) Drying of clothes in washing machine.

b) Cream separator.

c) Centrifugal governor.

d) Centrifuge machine used to separate solids from liquids.

1. Define linear motion.

Motion along a straight line is called linear motion.

2. Define circular motion. Give examples.

Motion along a circular path is called circular motion.

Eg:

i. A stone tied to a string and whirled around.

ii. A bicycle or a car moving in a circular path

iii. Moon revolving around the earth

iv. Merry-go-round

v. The giant wheel

vi. Compact disc in a player

vii. Motion of electrons around the atom

viii. Motion of the earth causing day and night

ix. Pulley used to draw water in a well

x. Movement of stars around the centre of a galaxy

3. Define uniform circular motion.

Motion along a circular path with constant speed is called uniform circular motion.

4. Define axis of rotation.

Axis of rotation is a straight line through all fixed points of a rotating rigid body around which all other points of the body move in circles.

Or

A three-dimensional object always rotates around an imaginary line called a rotation axis.

5. Define centrifugal reaction. Give examples.

Centrifugal reaction is a force equal and opposite of the centripetal force and is directed away from the center.

Eg:

i. When a stone tied to a string is whirled around it exerts an outward force namely the centrifugal reaction on the hand.

ii. The wheels of a train taking a curved path, exerts a lateral thrust on the outer rails. This lateral thrust is the centrifugal reaction.

6. Define centrifugal force.

Centrifugal force cannot be created. It is experienced only in the context of circular motion or rotational motion. Centrifugal force is due to the inertia of rotational motion. It is a pseudo force

7. Centrifugal force as no real existence. Justify.

There is a common misconception that an object moving in a circle has an outward force called centrifugal force acting on it. For a convincing evidence that ‘centrifugal force’ has no real existence, consider what happens when the string whirling a stone snaps. If a centrifugal force were acting, the ball would fly outwards. But it does not; the ball flies off tangentially because the inward force no longer acts.

8. Write some illustrations for centrifugal force(Applications).

a. centrifuge: Centrifuge is a device to separate minute particles of different densities.

b. centrifugal drying machine: A machine used for drying wet clothes. When the cylinder carrying the wet clothes rotates rapidly, water from the wet clothes is forced out due to the centrifugal force.

c. centrifugal pump: Centrifugal pump, used for transferring large quantities of liquids from low pressure regions,

d. centrifugal governor: it is an automatic device to control the speed of the engine.

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