#### Chapters

Chapter 2: Physics and Mathematics

Chapter 3: Rest and Motion: Kinematics

Chapter 4: The Forces

Chapter 5: Newton's Laws of Motion

Chapter 6: Friction

Chapter 7: Circular Motion

Chapter 8: Work and Energy

Chapter 9: Centre of Mass, Linear Momentum, Collision

Chapter 10: Rotational Mechanics

Chapter 11: Gravitation

Chapter 12: Simple Harmonics Motion

Chapter 13: Fluid Mechanics

Chapter 14: Some Mechanical Properties of Matter

Chapter 15: Wave Motion and Waves on a String

Chapter 16: Sound Waves

Chapter 17: Light Waves

Chapter 18: Geometrical Optics

Chapter 19: Optical Instruments

Chapter 20: Dispersion and Spectra

Chapter 21: Speed of Light

Chapter 22: Photometry

#### H.C. Verma Concept of Physics Part-1 (2018-2019 Session) by H.C Verma

## Chapter 7: Circular Motion

#### Chapter 7: Circular Motion Exercise Short Answers solutions [Pages 111 - 112]

You are driving a motorcycle on a horizontal road. It is moving with a uniform velocity. Is it possible to accelerate the motorcycle without putting higher petrol input rate into the engine?

Some washing machines have cloth driers. It contains a drum in which wet clothes are kept. As the drum rotates, the water particles get separated from the cloth. The general description of this action is that "the centrifugal force throws the water particles away from the drum". Comment on this statement from the viewpoint of an observer rotating with the drum and the observer who is washing the clothes.

A small coin is placed on a record rotating at \[33\frac{1}{3}\] rev/minute. The coin does not slip on the record. Where does it get the required centripetal force from ?

A bird while flying takes a left turn, where does it get the centripetal force from ?

Is it necessary to express all angles in radian while using the equation ω = ω_{0} + *at *?

After a good meal at a party you wash your hands and find that you have forgotten to bring your handkerchief. You shake your hand vigorously to remove the water as much as you can. Why is water removed in this process?

A smooth block loosely fits in a circular tube placed on a horizontal surface. The block moves in a uniform circular motion along the tube. Which wall (inner or outer) will exert a nonzero normal contact force on the block?

Consider the circular motion of the earth around the sun. Which of the following statements is more appropriate ?

(a) Gravitational attraction of the sun on the earth is equal to the centripetal force.

(b) Gravitational attraction of the sun on the earth is the centripetal force.

A car driver going at some speed v suddenly finds a wide wall at a distance r. Should he apply brakes or turn the car in a circle of radius r to avoid hitting the wall?

A heavy mass m is hanging from a string in equilibrium without breaking it. When this same mass is set into oscillation, the string breaks. Explain.

#### Chapter 7: Circular Motion Exercise MCQ solutions [Pages 112 - 113]

When a particle moves in a circle with a uniform speed

its velocity and acceleration are both constant

its velocity is constant but the acceleration changes

its acceleration is constant but the velocity changes

its velocity and acceleration both change.

Tow cars having masses m_{1} and m_{2} moves in circles of radii r_{1} and r_{2} respectively. If they complete the circle in equal time, the ratio of their angular speed ω_{1}/ω_{2} is

m

_{1}/m_{2 }r

_{1}/r_{2 }m

_{1}r_{1}/m_{2}/r_{2 }1.

A car moves at a constant speed on a road as shown in figure. The normal force by the road on the car N_{A} and N_{B} when it is at the points A and B respectively.

N

_{A}= N_{B}N

_{A}> N_{B }N

_{A}< N_{B }insufficient

A particle of mass m is observed from an inertial frame of reference and is found to move in a circle of radius r with a uniform speed v. The centrifugal force on it is

\[\frac{\text{mv}^2}{\text{r}}\] towards the centre

\[\frac{\text{mv}^2}{\text{r}}\] away from the centre

\[\frac{\text{mv}^2}{\text{r}}\] along the tangent through the particle

zero

A particle of mass m rotates with a uniform angular speed ω. It is viewed from a frame rotating about the Z-axis with a uniform angular speed ω_{0}. The centrifugal force on the particle is

mω

^{2}a\[\text{ m } \omega_0^2 \text{ a }\]

\[\text{ m } \left( \frac{\omega + \omega_0}{2} \right)^2 \text{a}\]

mω ω

_{0}a.

A particle is kept fixed on a turntable rotating uniformly. As seen from the ground the particle goes in a circle, its speed is 20 cm/s and acceleration is 20 cm/s^{2}. The particle is now shifted to a new position to make the radius half of the original value. The new value of the speed and acceleration will be

10 cm/s, 10 cm/s

^{2 }10 cm/s, 80 cm/s

^{2}40 cm/s, 10 cm/s

^{2}40 cm/s, 40 cm/s

^{2}

Water in a bucket is whirled in a vertical circle with string attached to it. The water does no fall down even when the bucket is inverted at the top of its path. We conclude that in this position

\[\text{mg }= \frac{\text{mv}^2}{\text{r}}\]

mg is greater than \[\frac{\text{mv}^2}{\text{r}}\]

mg is not greater than \[\frac{\text{mv}^2}{\text{r}}\]

mg is not less than \[\frac{\text{mv}^2}{\text{r}}\]

A stone of mass m tied to a string of length l is rotated in a circle with the other end of the string as the centre. The speed of the stone is v. If the string breaks, the stone will move

towards the centre

away from the centre

along a tangent

will stop.

A coin placed on a rotating turntable just slips. If it is placed at a distance of 4 cm from the centre. If the angular velocity of the turntable is doubled, it will just slip at a distance of

1 cm

2 cm

4 cm

8 cm

A motorcycle is going on an overbridge of radius *R*. The driver maintains a constant speed. As the motorcycle is ascending on the overbridge, the normal force on it

increases

decreases

remains the same

fluctuates.

Three identical cars A, B and C are moving at the same speed on three bridges. The car A goes on a place bridge, B on a bridge convex upward and C goes on a bridge concave upward. Let F_{A}, F_{B} and F_{C} be the normal forces exerted by the car on the bridges when they are at the middle of bridges.

F

_{A}is maximum of the three forces.F

_{B}is maximum of the three forces.F

_{C}is maximum of the three forces.F

_{A}= F_{B}_{ }= F_{C}.

A train A runs from east to west and another train B of the same mass runs from west to east at the same speed along the equator. A presses the track with a force F_{1} and B presses the track with a force F_{2}.

F

_{1}> F_{2}.F

_{1}< F_{2}F

_{1}= F_{2}the information is insufficient to find the relation between F

_{1}and F_{2}_{. }

If the earth stop rotating, the apparent value of g on its surface will

increase everywhere

decrease everywhere

remain the same everywhere

increase at some places and remain the same at some other places.

A rod of length L is pivoted at one end and is rotated with a uniform angular velocity in a horizontal plane. Let T_{1} and T_{2} be the tensions at the points L/4 and 3L/4 away from the pivoted ends.

T

_{1}> T_{2}T

_{2}> T_{1}T

_{1}= T_{2}The relation between T

_{1}and T_{2}depends on whether the rod rotates clockwise or anticlockwise.

A simple pendulum having a bob of mass m is suspended from the ceiling of a car used in a stunt film shooting. the car moves up along an inclined cliff at a speed v and makes a jump to leave the cliff and lands at some distance. Let R be the maximum height of the car from the top of the cliff. The tension in the string when the car is in air is

mg

\[\text{ mg} - \frac{\text{mv}^2}{\text{R}}\]

\[\text{mg} + \frac{\text{mv}^2}{\text{R}}\]

zero.

Let θ denote the angular displacement of a simple pendulum oscillating in a vertical plane. If the mass of the bob is m, the tension is the string is mg cos θ

always

never

at the extreme positions

at the mean position.

#### Chapter 7: Circular Motion Exercise MCQ solutions [Pages 113 - 114]

An object follows a curved path. The following quantities may remain constant during the motion

(a) speed

(b) velocity

(c) acceleration

(d) magnitude of acceleration.

Assume that the earth goes round the sun in a circular orbit with a constant speed of 30 kms

The average velocity of the earth from 1st jan, 90 to 30th June, 90 is zero.

The average acceleration during the above period is 60 km/s

^{2}.The average speed from 1st Jan, 90 to 31st Dec, 90 is zero.

The instantaneous acceleration of the earth points towards the sun.

The position vector of a particle in a circular motion about the origin sweeps out equal area in equal time. Its

(a) velocity remains constant

(b) speed remains constant

(c) acceleration remains constant

(d) tangential acceleration remains constant.

A particle is going in a spiral path as shown in figure with constant speed.

The velocity of the particle is constant.

The acceleration of the particle is constant.

The magnitude of acceleration is constant.

The magnitude of acceleration is decreasing continuously.

A car of mass M is moving on a horizontal circular path of radius r. At an instant its speed is v and is increasing at a rate a.

(a) The acceleration of the car is towards the centre of the path.

(b) The magnitude of the frictional force on the car is greater than \[\frac{\text{mv}^2}{\text{r}}\]

(c) The friction coefficient between the ground and the car is not less than a/g.

(d) The friction coefficient between the ground and the car is \[\mu = \tan^{- 1} \frac{\text{v}^2}{\text{rg}.}\]

A circular road of radius r is banked for a speed v = 40 km/hr. A car of mass of m attempts to go on the circular road. The friction coefficient between the tyre and the road is negligible.

(a) The are cannot make a turn without skidding.

(b) If the car turns at a speed less than 40 km/hr, it will slip down.

(c) If the car turns at the car is equal to \[\frac{\text{mv}^2}{\text{r}}\]

(d) If the car turns at the correct speed of 40 km/hr, the force by the road on the car is greater than mg as well as greater than \[\frac{\text{mv}^2}{\text{r}}\]

A person applies constant force \[\overrightarrow{\text{F}}\]on a particle of mass *m* and finds that the particle moves in a circle of radius *r* with a uniform speed *v* as seen from an inertial frame of reference.

(a) This is not possible.

(b) There are other forces on the particle.

(c) The resultant of the other forces is \[\frac{\text{mv}^2}{\text{r}}\] towards the centre.

d) The resultant of the other forces varies in magnitude as well as in direction

#### Chapter 7: Circular Motion solutions [Pages 114 - 116]

Find the acceleration of the moon with respect to the earth from the following data : Distance between the earth and the moon = 3.85 × 10^{5} km and the time taken by the moon to complete one revolution around the earth = 27.3 days.

Find the acceleration of a particle placed on the surface of the earth at the equator due to earth's rotation. The diameter of earth = 12800 km and it takes 24 hours for the earth to complete one revolution about its axis.

A particle moves in a circle of radius 1.0 cm at a speed given by *v* = 2.0 *t* where *v* is cm/s and *t* in seconds.

(a) Find the radial acceleration of the particle at *t* = 1 s.

(b) Find the tangential acceleration at *t* = 1 s.

(c) Find the magnitude of the acceleration at *t* = 1 s.

A scooter weighing 150 kg together with its rider moving at 36 km/hr is to take a turn of a radius 30 m. What horizontal force on the scooter is needed to make the turn possible ?

If the horizontal force needed for the turn in the previous problem is to be supplied by the normal force by the road, what should be the proper angle of banking?

A park has a radius of 10 m. If a vehicle goes round it at an average speed of 18 km/hr, what should be the proper angle of banking?

If the road of the previous problem is horizontal (no banking), what should be the minimum friction coefficient so that scooter going at 18 km/hr does not skid?

A circular road of radius 50 m has the angle of banking equal to 30°. At what speed should a vehicle go on this road so that the friction is not used?

In the Bohr model of hydrogen atom, the electron is treated as a particle going in a circle with the centre at the proton. The proton itself is assumed to be fixed in an inertial frame. The centripetal force is provided by the Coulomb attraction. In the ground state, the electron goes round the proton in a circle of radius 5.3 × 10^{−11} m. Find the speed of the electron in the ground state. Mass of the electron = 9.1 × 10^{−31} kg and charge of the electron = 1.6 × 10^{−19} C.

A stone is fastened to one end of a string and is whirled in a vertical circle of radius *R*. Find the minimum speed the stone can have at the highest point of the circle.

A ceiling fan has a diameter (of the circle through the outer edges of the three blades) of 120 cm and rpm 1500 at full speed. Consider a particle of mass 1 g sticking at the outer end of a blade. How much force does it experience when the fan runs at full speed? Who exerts this force on the particle? How much force does the particle exert on the blade along its surface?

A mosquito is sitting on an L.P. record disc rotating on a turn table at \[33\frac{1}{3}\] revolutions per minute. The distance of the mosquito from the centre of the turn table is 10 cm. Show that the friction coefficient between the record and the mosquito is greater than π^{2}/81. Take g =10 m/s^{2}.

A simple pendulum is suspended from the ceiling of a car taking a turn of radius 10 m at a speed of 36 km/h. Find the angle made by he string of the pendulum with the vertical if this angle does not change during the turn. Take g = 10 m/s^{2}.

The bob of a simple pendulum of length 1 m has mass 100 g and a speed of 1.4 m/s at the lowest point in its path. Find the tension in the string at this instant.

Suppose the bob of the previous problem has a speed of 1.4 m/s when the string makes an angle of 0.20 radian with the vertical. Find the tension at this instant. You can use cos θ ≈ 1 − θ^{2}/2 and SINθ ≈ θ for small θ.

Suppose the amplitude of a simple pendulum having a bob of mass m is θ_{0}. Find the tension in the string when the bob is at its extreme position.

A person stands on a spring balance at the equator. By what fraction is the balance reading less than his true weight?

A person stands on a spring balance at the equator. If the speed of earth's rotation is increased by such an amount that the balance reading is half the true weight, what will be the length of the day in this case?

A turn of radius 20 m is banked for the vehicles going at a speed of 36 km/h. If the coefficient of static friction between the road and the tyre is 0.4, what are the possible speeds of a vehicle so that it neither slips down nor skids up?

A motorcycle has to move with a constant speed on an over bridge which is in the form of a circular arc of radius R and has a total length L. Suppose the motorcycle starts from the highest point.(a) What can its maximum velocity be for which the contact with the road is not broken at the highest point? (b) If the motorcycle goes at speed 1/√2 times the maximum found in part (a), where will it lose the contact with the road? (c) What maximum uniform speed can it maintain on the bridge if it does not lose contact anywhere on the bridge?

A car goes on a horizontal circular road of radius *R*, the speed increasing at a constant rate \[\frac{\text{dv}}{\text{dt}} = a\] . The friction coefficient between the road and the tyre is μ. Find the speed at which the car will skid.

A block of mass m is kept on a horizontal ruler. The friction coefficient between the ruler and the block is μ. The ruler is fixed at one end and the block is at a distance L from the fixed end. The ruler is rotated about the fixed end in the horizontal plane through the fixed end. (a) What can the maximum angular speed be for which the block does not slip? (b) If the angular speed of the ruler is uniformly increased from zero at an angular acceleration α, at what angular speed will the block slip?

A track consists of two circular parts ABC and CDE of equal radius 100 m and joined smoothly as shown in figure. Each part subtends a right angle at its centre. A cycle weighing 100 kg together with the rider travels at a constant speed of 18 km/h on the track. (a) Find the normal contact force by the road on the cycle when it is at B and at D. (b) Find the force of friction exerted by the track on the tyres when the cycle is at B, C and. (c) Find the normal force between the road and the cycle just before and just after the cycle crosses C. (d) What should be the minimum friction coefficient between the road and the tyre, which will ensure that the cyclist can move with constant speed? Take g = 10 m/s^{2}.

In a children's park a heavy rod is pivoted at the centre and is made to rotate about the pivot so that the rod always remains horizontal. Two kids hold the rod near the ends and thus rotate with the rod (In the following figure). Let the mass of each kid be 15 kg, the distance between the points of the rod where the two kids hold it be 3.0 m and suppose that the rod rotates at the rate of 20 revolutions per minute. Find the force of friction exerted by the rod on one of the kids.

A hemispherical bowl of radius R is rotated about its axis of symmetry which is kept vertical. A small block is kept in the bowl at a position where the radius makes an angle θ with the vertical. The block rotates with the bowl without any slipping. The friction coefficient between the block and the bowl surface is μ. Find the range of the angular speed for which the block will not slip.

A particle is projected with a speed *u* at an angle θ with the horizontal. Consider a small part of its path near the highest position and take it approximately to be a circular arc. What is the radius of this circular circle? This radius is called the radius of curvature of the curve at the point.

What is the radius of curvature of the parabola traced out by the projectile in the previous problem at a point where the particle velocity makes an angle θ/2 with the horizontal?

A block of mass m moves on a horizontal circle against the wall of a cylindrical room of radius R. The floor of the room on which the block moves is smooth but the friction coefficient between the wall and the block is μ. The block is given an initial speed v_{0}. As a function of the speed v write (a) the normal force by the wall on the block, (b) the frictional force by wall and (c) the tangential acceleration of the block. (d) Integrate the tangential acceleration \[\left( \frac{dv}{dt} = v\frac{dv}{ds} \right)\] to obtain the speed of the block after one revolution.

A table with smooth horizontal surface is fixed in a cabin that rotates with a uniform angular velocity ω in a circular path of radius R (In the following figure). A smooth groove AB of length L(<<R) is made the surface of the table. The groove makes an angle θ with the radius OA of the circle in which the cabin rotates. A small particle is kept at the point A in the groove and is released to move at the point A in the groove and is released to move along AB. Find the time taken by the particle to reach the point B.

A car moving at a speed of 36 km/hr is taking a turn on a circular road of radius 50 m. A small wooden plate is kept on the seat with its plane perpendicular to the radius of the circular road (In the following figure). A small block of mass 100 g is kept on the seat which rests against the plate. the friction coefficient between the block and the plate is. (a) Find the normal contact force exerted by the plate on the block. (b) The plate is slowly turned so that the angle between the normal to the plate and the radius of the road slowly increases. Find the angle at which the block will just start sliding on the plate.

A table with smooth horizontal surface is placed in a circle of a large radius R (In the following figure). A smooth pulley of small radius is fastened to the table. Two masses m and 2m placed on the table are connected through a string going over the pulley. Initially the masses are held by a person with the string along the outward radius and then the system is released from rest (with respect to the cabin). Find the magnitude of the initial acceleration of the masses as seen from the cabin and the tension in the string.

## Chapter 7: Circular Motion

#### H.C. Verma Concept of Physics Part-1 (2018-2019 Session) by H.C Verma

#### Textbook solutions for Class 12

## H.C. Verma solutions for Class 12 Physics chapter 7 - Circular Motion

H.C. Verma solutions for Class 12 Physics chapter 7 (Circular Motion) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Concept of Physics Part-1 (2018-2019 Session) by H.C Verma solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Class 12 Physics chapter 7 Circular Motion are Lubrication - (Laws of Motion), Law of Conservation of Linear Momentum and Its Applications, Aristotle’s Fallacy, The Law of Inertia, Newton'S First Law of Motion, Newton’s Second Law of Motion, Newton's Third Law of Motion, Conservation of Momentum, Equilibrium of a Particle, Common Forces in Mechanics, Circular Motion, Solving Problems in Mechanics, Static and Kinetic Friction, Laws of Friction, Inertia, Intuitive Concept of Force, Dynamics of Uniform Circular Motion - Centripetal Force, Examples of Circular Motion (Vehicle on a Level Circular Road, Vehicle on a Banked Road), Rolling Friction.

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