Circular Motion–9th Standard Science Notes and Question Bank

Circular Motion

Text Book Question and Answers

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 centripetal 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)

IV. Answer the following questions.

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.

Additional Questions and Answers

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.
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.

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.
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.
9. When a mixture of fine sand, sawdust and water are rotated in a centrifuge, sand moves away towards the edges. Why? 
In the mixture, sand is heavier and experiences larger centrifugal reaction.
10. A person wrongly purchases a centrifugal governor to transfer a liquid from a low pressure region. Name the device that the person should have purchased instead of centrifugal governor.

Centrifugal pump.
11.Why does a runner while running in a sharp curved path lean inward?

To obtain the necessary centripetal force.
12.Under what conditions does an object move along a straight line?

For an object to move in a straight line, the net force acting on it should be zero.
13.State the acceleration experienced by moon?

The acceleration experienced by moon is 2.72x10-3 m/s2.
14.What is the cause for centrifugal force?

Centrifugal force is caused due to inertia of rotational motion.
15.Write two applications of centrifugal pumps?

Centrifugal pumps are used in blowers and exhaust fans.
16.How are railway trucks laid in curves?

In a curve, the outer rail is raised a little above the inner one to prevent wear and tear on the wheels and to avoid the train going off the track
17.Water and castor oil mixture is separated in a centrifuge. Where does the oil settle down?

Caster oil being heavier than water settles away from the centre

Choose the correct answers

18. When creamy curd is churned in a centrifuge the butter particles congregate at the centre of the rotating medium. This is because
A] butter particles experience greater centrifugal reaction
B] butter particles experience less centrifugal reaction
C] butter particles experience no centrifugal reaction
D] butter particles and the particles of the medium experience the same centrifugal reaction.
19. A cylindrical vessel containing a mixture of water, rice and pieces of stone is rotated rapidly about its axis. Then
A] rice is seen moving away from the axis
B] pieces of stone move away from the axis
C] pieces of stone remain closer to the axis
D] both the stone and rice remain near the axis.
20. The device used in separating proteins, hormones, viruses from a liquid medium is
A] Fractionating column
B] Centrifugal governor
C] Centrifuge
D] Filtration apparatus
21. Which one of the following is an application of centrifugal reaction?
A] Separating butter from curds with a churner
B] Separation of petroleum products
C] Concentration of haematite ore
D] Drying clothes in sun.
22. If a string whirling with a stone snap, then the motion of the stone will be
A] in the direction of the circular motion
B] towards the centre of the circular path
C] tangential to the circular path
D] opposite to the direction of the circular path.
23. A cyclist while going round a curve leans towards the centre of the curve to get necessary
A] centrifugal force
B] centripetal force
C] gravitational force
D] centrifugal reaction
24. The device that can be fitted to the engine of a bus to prevent the
driver from driving it with overspeed is
A] centrifugal pump
B] centrifuge
C] centrifugal drying machine
D] centrifugal governor
25. The function of Centrifugal Governor is to
A] protect the engine from short circuit
B] prevent the engine from overheating
C] control the speed of the engine
D] stop the engine
26. The passengers inside an automobile moving fast along a curve feel pushed outwards due to
A] centripetal force
B] centripetal reaction
C] centrifugal reaction
D] centripetal acceleration
27. 'When we are moving in a cycle which is negotiating a curve, our body leans towards the centre of the curve’. The reason is
A] Centrifugal reaction
B] Centripetal force
C] Gravitational force
D] Force of the cycle
28. In rail tracks the outer track is raised in a curve a little above the inner rail track. This reduces
A] velocity
B] wear and tear
C] inertia
D] acceleration
29. Which one of the following is responsible for separation of particles of different densities in a centrifuge?
A] Centrifugal force
B] Centrifugal reaction
C] Centripetal force
D] Centripetal reaction
30. If a string whirling with a stone snap, then the motion of stone will be
A] in same circular motion
B] towards centre
C] in the direction of tangent
D] in the extending (expanding) circular path
31. The device that separates proteins from a liquid media is
A] centrifuge
B] microwave-oven
C] refrigerator
D] amplifier
32. Motion along a circular path with constant speed is
A] uniform circular motion
B] uniform linear motion
C] variable acceleration
D] retarded motion
33. An astronaut experiences weightlessness while a spacecraft orbiting around the earth. It is due to
A] the weight of the spacecraft
B] distance to the spacecraft from the earth
C] no gravitational force on astronaut
D] absence of air in the space
34. A stone tied to a string is given circular motion. The inertia of rotational motion produces
A] centripetal force
B] centrifugal reaction
C] centripetal acceleration
D] centrifugal acceleration
35. The device used to separate the viruses from a liquid media is
A] spectroscope
B] ultrasound scanner
C] sonar
D] centrifuge

Fill in the blanks

36. The velocity of a body moving in a circular path will be along the direction of Tangent to the circle.
37. The force acting on a body moving in a circular path towards the centre of the circle is called Centripetal force.
38. The outer edge of a curved road is raised over the inner edge, which is called banking of roads.
39. If a denser liquid ‘A’ and a less dense liquid ‘B’ is subjected to centrifugation, the larger centrifugal reaction is experienced by liquid ‘A’
40. The speed of a vehicle can be controlled by the device called centrifugal governor
41. Virus and bacteria can be separated from a liquid medium a device called centrifuge
42. Circular motion is called continuously accelerated motion because the direction of velocity changes at every point on the circle.
43. If the average velocity of a body of mass 1kg moving in a circular path is 2m/s and the radius of the circular path 2m, its centripetal force is 2 Newtons.
44. A passenger travelling in a bus feels pushed away from the centre of the curve if the bus moves in a circular path due to centrifugal reaction.
45. The centripetal force to keep the moon orbiting around the earth is provided by earth’s gravity

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