# NCERT Physics Class 11 Exemplar Ch 7 System Of Particles And Rotational Motion Part 4

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## One or More then Answer

Q.13. With reference to Fig. of a cube of edge a and mass m, state whether the following are true or false. (O is the center of the cube.)

The moment of inertia of cube about z-axis is

Answer: False, perpendicular axis theorem is applicable only to a lamina.

(b) The moment of inertia of cube about z ′ is

Answer: True

(C)The moment of inertia of cube about z″ is

Answer: False, z and z” are not parallel axes.

(d)

Answer: True.

## Very Short Answer

Q.14 The center of gravity of a body on the earth coincides with its center of mass for a ‘small’ object whereas for an ‘extended’ object it may not. What is the qualitative meaning of ‘small’ and ‘extended’ in this regard?

For which of the following the two coincides? A building, a pond, a lake, a mountain?

Answer: When the vertical height of the object is very small as compared to earth’s radius, we call the object small, otherwise it is extended.

(a) Building and pond are small objects.

(b) A deep lake and a mountain are examples of extended objects.

Q.15 Why does a solid sphere have smaller moment of inertia than a hollow cylinder of same mass and radius, about an axis passing through their axes of symmetry?

Answer: All the mass in a cylinder lies at distance R from the axis of symmetry but most of the mass of a solid sphere lies at a smaller distance than R.

Q.16 The variation of angular position , of a point on a rotating rigid body, with time t is shown in Fig. Is the body rotating clock-wise or anti-clockwise?

Answer: Positive slope indicates anticlockwise rotation which is traditionally

taken as positive