<< Previous Chapter 5 : Work, Energy And Power
Next Chapter 7 : Gravitation >>

###
**Exercise : ** Solutions of Questions on Page Number : **178**

<< Previous Chapter 5 : Work, Energy And Power
Next Chapter 7 : Gravitation >>
**Popular Articles**

Q1 :
**
**

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?

**Answer :**

Geometric centre; No

The centre of mass (C.M.) is a point where the mass of a body is supposed to be concentrated. For the given geometric shapes having a uniform mass density, the C.M. lies at their respective geometric centres.

The centre of mass of a body need not necessarily lie within it. For example, the C.M. of bodies such as a ring, a hollow sphere, etc., lies outside the body.

Answer needs Correction? Click Here

Q2 :
**
**

In the HCl molecule, the separation between the nuclei of the two
atoms is about 1.27 Ãƒâ€¦ (1
Ãƒâ€¦ =
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.

**Answer :**

The given situation can be shown as:

Distance between H and Cl atoms = 1.27Ãƒâ€¦

Mass of H atom = *m*

Mass of Cl atom = 35.5*m*

Let the centre of mass of the system lie at a distance *x*
from the Cl atom.

Distance of the centre of mass from the H atom = (1.27
â€“ *x*)

Let us assume that the centre of mass of the given molecule lies at the origin. Therefore, we can have:

Here, the negative sign indicates that the centre of mass lies at the left of the molecule. Hence, the centre of mass of the HCl molecule lies 0.037Ãƒâ€¦ from the Cl atom.

Answer needs Correction? Click Here

Q3 :
**
**

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?

**Answer :**

Q4 :
**
**

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

**Answer :**

Q5 :
**
**

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

**Answer :**

Q6 :
**
**

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
*p*_{x},
*p*_{y}
and
*p*_{z}. Show
that if the particle moves only in the
*x*-*y*
plane the angular momentum has only a
*z*-component.

**Answer :**

Q7 :
**
**

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.

**Answer :**

Q8 :
**
**

A non-uniform bar of weight *W*
is suspended at rest by two strings of negligible weight as
shown in Fig.7.39. 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.

**Answer :**

Q9 :
**
**

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.

**Answer :**

Q10 :
**
**

(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
2*MR*^{2}/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
*MR*^{2}/4, find its
moment of inertia about an axis normal to the disc and passing
through a point on its edge.

**Answer :**

Q11 :
**
**

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?

**Answer :**

Q12 :
**
**

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?

**Answer :**

Q13 :
**
**

(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?

**Answer :**

Q14 :
**
**

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.

**Answer :**

Q15 :
**
**

To maintain a rotor at a uniform angular speed of 200 rad
s^{-1}, an engine needs to transmit a
torque of 180 Nm. 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.

**Answer :**

Q16 :
**
**

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.

**Answer :**

Q17 :
**
**

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?

**Answer :**

Q18 :
**
**

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?

**Answer :**

Q19 :
**
**

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?

**Answer :**

Q20 :
**
**

The oxygen molecule has a mass of 5.30
x
10^{-26} kg
and a moment of inertia of
1.94 x 10^{-46}
kg m^{2} 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.

**Answer :**

Q21 :
**
**

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?

**Answer :**

Q22 :
**
**

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/s^{2})

(Hint: Consider the equilibrium of each side of the ladder separately.)

**Answer :**

Q23 :
**
**

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 to 7.6 kg
m^{2}.

(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?

**Answer :**

Q24 :
**
**

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
*ML*^{2}/3.)

**Answer :**

Q25 :
**
**

Two discs of moments of inertia *I*_{1} and
*I*_{2} about their respective axes (normal to the
disc and passing through the centre), and rotating with angular
speeds Ãâ€°_{1} and
Ãâ€°_{2} 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 Ãâ€°_{1}
≠
Ãâ€°_{2}.

**Answer :**

Q26 :
**
**

(a) Prove the theorem of perpendicular axes.

(Hint: Square of the distance of a point
(*x, y*) in the
*xâ€“y* plane from an
axis through the origin perpendicular to the plane is
*x*^{2} *+
y*^{2}).

(b) Prove the theorem of parallel axes.

(Hint: If the centre of mass is chosen to be
the origin **)**.

**Answer :**

Q27 :
**
**

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 .

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.

**Answer :**

Q28 :
**
**

A disc rotating about its axis with angular speed
Ã”°_{o}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?

**Answer :**

Q29 :
**
**

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?

**Answer :**

Q30 :
**
**

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 *ÃŽÂ¼*_{k} =
0.2.

**Answer :**

Q31 :
**
**

A cylinder of mass 10 kg and radius 15 cm is rolling
perfectly on a plane of inclination
30°. The
coefficient of static friction
*µ*_{s}=
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
*ÃŽÂ¸*of the
plane is increased, at what value of
*ÃŽÂ¸*
does the cylinder begin to skid, and not roll
perfectly?

**Answer :**

Q32 :
**
**

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.

**Answer :**

Q33 :
**
**

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**_{i}
=
**p**'_{i}
+
*m*_{i}**V**

Where
**p**_{i} is
the momentum of the
*i*^{th} particle
(of mass
*m*_{i})
and
**p**“²
_{i} =
*m*_{i}
**v**“²
_{i}. Note
**v**“²
_{i} is the velocity of
the *i*^{th}
particle relative to the centre of mass.

Also, prove using the definition of the centre of mass

(b) Show *K* =
*K*“²
+
½*MV*^{2}

Where *K* is the total
kinetic energy of the system of particles,
*K*“²
is the total kinetic energy of the system when the particle
velocities are taken with respect to the centre of mass
and
*MV*^{2}/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 r

**Answer :**

Physics Part-1 - Physics : CBSE ** NCERT ** Exercise Solutions for Class 11th for ** System Of Particles And Rotational Motion ** will be available online in PDF book form soon. The solutions are absolutely Free. Soon you will be able to download the solutions.

- Physics Part-1 : Chapter 3 - Motion In A Plane Class 11
- Physics Part-1 : Chapter 1 - Units And Measurements Class 11
- Physics Part-1 : Chapter 2 - Motion In A Straight Line Class 11
- Physics Part-1 : Chapter 4 - Laws Of Motion Class 11
- Physics Part-2 : Chapter 1 - Mechanical Properties Of Solids Class 11
- Physics Part-1 : Chapter 7 - Gravitation Class 11
- Physics Part-1 : Chapter 5 - Work, Energy And Power Class 11
- Physics Part-2 : Chapter 7 - Waves Class 11
- Physics Part-2 : Chapter 4 - Thermodynamics Class 11