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**Question 7.1** Give the location of the centre of mass of a
(i) sphere, (ii) cylinder, (iii) ring, and (iv)
cube, each of uniform mass density. Does the centre of mass of a body necessarily
lie inside the body ?

**Question 7.2** In the HC1 molecule, the separation between the nuclei of the two atoms is about
1.27 Å (1 Å = 10-10 m). Find the approximate location of the CM of the molecule,
given that a chlorine atom is about 35.5 times as massive as a hydrogen atom and
nearly all the mass of an atom is concentrated in its nucleus.

**Question 7.3 **A child sits stationary at one end of a long trolley moving uniformly with a speed V
on a smooth horizontal floor. If the child gets up and runs about on the trolley in any
manner, what is the speed of the CM of the (trolley + child) system ?

**Question 7.4** Show that the area of the triangle contained between the vectors a and b is one half
of the magnitude of a × b.

**Question 7.5** Show that a.(b × c) is equal in magnitude to the volume of the parallelepiped formed
on the three vectors , a, b and c.

**Question 7.6 **Find the components along the x, y, z axes of the angular momentum l of a particle,
whose position vector is r with components x, y, z and momentum is p with
components px, py and pz. Show that if the particle moves only in the x-y plane the
angular momentum has only a z-component.

**Question 7.7** Two particles, each of mass m and speed v, travel in opposite directions along parallel
lines separated by a distance d. Show that the vector angular momentum of the two
particle system is the same whatever be the point about which the angular momentum
is taken.

**Question 7.8** A non-uniform bar of weight W is suspended at rest by two strings of negligible
weight as shown in Fig.

**Question 7.9**. The angles made by the strings with the vertical are
36.9° and 53.1° respectively. The bar is 2 m long. Calculate the distance d of the
centre of gravity of the bar from its left end.7.9 A car weighs 1800 kg. The distance between its front and back axles is 1.8 m. Its
centre of gravity is 1.05 m behind the front axle. Determine the force exerted by the
level ground on each front wheel and each back wheel.

**Question 7.10** (a) Find the moment of inertia of a sphere about a tangent to the sphere, given the
moment of inertia of the sphere about any of its diameters to be 2MR2/5, where
M is the mass of the sphere and R is the radius of the sphere.

(b) Given the moment of inertia of a disc of mass M and radius R about any of its
diameters to be MR2/4, find its moment of inertia about an axis normal to the
disc and passing through a point on its edge.

**Question 7.11** Torques of equal magnitude are applied to a hollow cylinder and a solid sphere, both
having the same mass and radius. The cylinder is free to rotate about its standard
axis of symmetry, and the sphere is free to rotate about an axis passing through its
centre. Which of the two will acquire a greater angular speed after a given time.

**Question 7.12** A solid cylinder of mass 20 kg rotates about its axis with angular speed 100 rad s-1.
The radius of the cylinder is 0.25 m. What is the kinetic energy associated with the
rotation of the cylinder? What is the magnitude of angular momentum of the cylinder
about its axis?

**Question 7.13** (a) A child stands at the centre of a turntable with his two arms outstretched. The
turntable is set rotating with an angular speed of 40 rev/min. How much is the
angular speed of the child if he folds his hands back and thereby reduces his
moment of inertia to 2/5 times the initial value ? Assume that the turntable
rotates without friction.

(b) Show that the child’s new kinetic energy of rotation is more than the initial
kinetic energy of rotation. How do you account for this increase in kinetic energy?

**Question 7.14** A rope of negligible mass is wound round a hollow cylinder of mass 3 kg and radius
40 cm. What is the angular acceleration of the cylinder if the rope is pulled with a
force of 30 N ? What is the linear acceleration of the rope ? Assume that there is no
slipping.

**Question 7.15** To maintain a rotor at a uniform angular speed or 200 rad s-1, an engine needs to
transmit a torque of 180 N m. What is the power required by the engine ?
(Note: uniform angular velocity in the absence of friction implies zero torque. In
practice, applied torque is needed to counter frictional torque). Assume that the
engine is 100% efficient.

**Question 7.16** From a uniform disk of radius R, a circular hole of radius R/2 is cut out. The centre
of the hole is at R/2 from the centre of the original disc. Locate the centre of gravity
of the resulting flat body.

**Question 7.17** A metre stick is balanced on a knife edge at its centre. When two coins, each of mass
5 g are put one on top of the other at the 12.0 cm mark, the stick is found to be
balanced at 45.0 cm. What is the mass of the metre stick?

**Question 7.18** A solid sphere rolls down two different inclined planes of the same heights but
different angles of inclination.

(a) Will it reach the bottom with the same speed in
each case?

(b) Will it take longer to roll down one plane than the other?

(c) If so,
which one and why?

**Question 7.19** A hoop of radius 2 m weighs 100 kg. It rolls along a horizontal floor so that its centre
of mass has a speed of 20 cm/s. How much work has to be done to stop it?

**Question 7.20** The oxygen molecule has a mass of 5.30 × 10-26 kg and a moment of inertia of
1.94×10-46 kg m2 about an axis through its centre perpendicular to the lines joining
the two atoms. Suppose the mean speed of such a molecule in a gas is 500 m/s and
that its kinetic energy of rotation is two thirds of its kinetic energy of translation.
Find the average angular velocity of the molecule.

**Question 7.21** A solid cylinder rolls up an inclined plane of angle of inclination 30° . At the bottom
of the inclined plane the centre of mass of the cylinder has a speed of 5 m/s.

(a) How far will the cylinder go up the plane?

(b) How long will it take to return to the bottom?
Additional Exercises

**Question 7.22** As shown in Fig.7.40, the two sides of a step ladder BA and CA are 1.6 m long and
hinged at A. A rope DE, 0.5 m is tied half way up. A weight 40 kg is suspended from
a point F, 1.2 m from B along the ladder BA. Assuming the floor to be frictionless
and neglecting the weight of the ladder, find the tension in the rope and forces
exerted by the floor on the ladder. (Take g = 9.8 m/s2)
(Hint: Consider the equilibrium of each side of the ladder separately.)

**Question 7.23 **A man stands on a rotating platform, with his arms stretched horizontally holding a
5 kg weight in each hand. The angular speed of the platform is 30 revolutions per
minute. The man then brings his arms close to his body with the distance of each
weight from the axis changing from 90cm to 20cm. The moment of inertia of the
man together with the platform may be taken to be constant and equal to7.6 kg m2.

(a) What is his new angular speed? (Neglect friction.)

(b) Is kinetic energy conserved in the process? If not, from where does the change
come about?

**Question 7.24** A bullet of mass 10 g and speed 500 m/s is fired into a door and gets embedded
exactly at the centre of the door. The door is 1.0 m wide and weighs 12 kg. It is
hinged at one end and rotates about a vertical axis practically without friction. Find
the angular speed of the door just after the bullet embeds into it.
(Hint: The moment of inertia of the door about the vertical axis at one end is ML2/3.)

**Question 7.25** Two discs of moments of inertia I1 and I2 about their respective axes (normal to the
disc and passing through the centre), and rotating with angular speeds ω1 and ω2
are brought into contact face to face with their axes of rotation coincident.

(a) What
is the angular speed of the two-disc system?

(b) Show that the kinetic energy of the
combined system is less than the sum of the initial kinetic energies of the two discs.
How do you account for this loss in energy? Take ω1 ≠ ω2.

**Question 7.26** (a) Prove the theorem of perpendicular axes.
(Hint : Square of the distance of a point (x, y) in the x–y plane from an axis through
the origin perpendicular to the plane is x2+y2).
(b) Prove the theorem of parallel axes.
(Hint : If the centre of mass is chosen to be the origin Σmi ri = 0 ).

**Question 7.27** Prove the result that the velocity v of translation of a rolling body (like a ring, disc,
cylinder or sphere) at the bottom of an inclined plane of a height h is given by
( )
2
2 2
2
1 /
gh
v
k R
=
+
using dynamical consideration (i.e. by consideration of forces and torques). Note k is
the radius of gyration of the body about its symmetry axis, and R is the radius of the
body. The body starts from rest at the top of the plane.

**Question 7.28** A disc rotating about its axis with angular speed ωo is placed lightly (without any
translational push) on a perfectly frictionless table. The radius of the disc is R. What
are the linear velocities of the points A, B and C on the disc shown in
Fig.7.41? Will the disc roll in the direction indicated ?

**Question 7.29** Explain why friction is necessary to make the disc in Fig. 7.41 roll in the direction indicated.

(a) Give the direction of frictional force at B, and the sense of frictional torque, before perfect rolling
begins.

(b) What is the force of friction after perfect rolling begins ?

**Question 7.30** A solid disc and a ring, both of radius 10 cm are placed on a horizontal table
simultaneously, with initial angular speed equal to 10 π rad s-1. Which of the two will start to roll
earlier ? The co-efficient of kinetic friction is μ
k
= 0.2.

**Question 7.31** A cylinder of mass 10 kg and radius 15 cm is rolling perfectly on a plane of inclination 30o. The coefficient
of static friction μs = 0.25.

(a) How much is the force of friction acting on the cylinder ?

(b) What is the work done against friction during rolling ?

(c) If the inclination θ of the plane is increased, at what value of θ does the cylinder begin to skid,
and not roll perfectly ?

**Question 7.32** Read each statement below carefully, and state, with reasons, if it is true or false

(a) During rolling, the force of friction acts in the same direction as the direction of motion of the CM
of the body.

(b) The instantaneous speed of the point of contact during rolling is zero.

(c) The instantaneous acceleration of the point of contact during rolling is zero.

(d) For perfect rolling motion, work done against friction is zero.

(e) A wheel moving down a perfectly frictionless inclined plane will undergo slipping (not rolling)
motion.

**Question 7.33** Separation of Motion of a system of particles into motion of the centre of mass and motion about the
centre of mass :

(a) Show p = pi +miV
where pi is the momentum of the ith particle (of mass mi) and p i = mi v i. Note v i is the velocity of
the ith particle relative to the centre of mass.
Also, prove using the definition of the centre of mass 0 iΣ p′ =

(b) Show K = K +½MV 2
where K is the total kinetic energy of the system of particles, K′ is the total kinetic energy of the
system when the particle velocities are taken with respect to the centre of mass and MV2/2 is the
kinetic energy of the translation of the system as a whole (i.e. of the centre of mass motion of the
system). The result has been used in Sec.7.14.

(c) Show
L = L′ + R × MV
where L = i i Σr × p′ is the angular momentum of the system about the centre of mass with
velocities taken relative to the centre of mass. Remember – i i r = r R ; rest of the notation is thestandard notation used in the chapter. Note L′ and MR × V can be said to be
angular momenta, respectively, about and of the centre of mass of the system of
particles.

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- Chapter 3: Motion in a Straight Line
**[Ques wise Ans]** - Chapter 1 Physical World
- Chapter 2 Units and Measurement
- Chapter 3 Motion in a Straight Line
- Chapter 4 Motion in a Plane
- Chapter 5 Laws of Motion
- Chapter 7 Systems of Particles and Rotational Motion
- Chapter 6 Work Energy and Power
- Chapter 8 Gravitation
- Chapter 9 Mechanical Properties of Solids
- Chapter 10 Mechanical Properties of Fluids
- Chapter 11 Thermal Properties of Matter
- Chapter 12 Thermodynamics
- Chapter 13 Kinetic Theory
- Chapter 14 Oscillations
- Chapter 15 Waves

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