NCERT Physics Class 11 Exemplar Ch 7 System Of Particles And Rotational Motion Part 3

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Q.9. Choose the correct alternatives:

(a) For a general rotational motion, angular momentum L and angular velocity need not be parallel.

(b) For a rotational motion about a fixed axis, angular momentum L and angular velocity are always parallel.

(c) For a general translational motion, momentum and velocity are always parallel.

(d) For a general translational motion, acceleration a and velocity v are always parallel.

Answer: (a), (c)

Q.10 Figure shows two identical particles 1 and 2, each of mass , moving in opposite directions with same speed v along parallel lines. At a particular instant, are their respective position vectors drawn from point A which is in the plane of the parallel lines. Choose the correct options:

Angular momentum of particle

Angular Momentum of Particle

(a) Angular momentum of particle 1 about ©

(b) Angular momentum of particle 2 about A is ©

(c) Total angular momentum of the system about A is ©

(d) Total angular momentum of the system about A is

© represents a unit vector coming out of the page.

⊗ represents a unit vector going into the page

Answer: (a), (d)

Q.11.The net external torque on a system of particles about an axis is zero. Which of the following are compatible with it?

(a) The forces may be acting radially from a point on the axis.

(b) The forces may be acting on the axis of rotation.

(c) The forces may be acting parallel to the axis of rotation.

(d) The torque caused by some forces may be equal and opposite to that caused by other forces.

Answer: All are true

Q. 12 Figure shows a lamina in x-y plane. Two axes pass perpendicular to its plane. A force F acts in the plane of lamina at point P as shown. Which of the following are true? (The point P is closer to z ′ -axis than the z-axis.)

Lamina in xy plane

Lamina in Xy Plane

(a) Torque caused by about z axis is along .

Answer: False, it is along

(b) Torque ′ caused by F about z ′ axis is along .

Answer: True

(c) Torque caused by F about z axis is greater in magnitude than that about z axis.

Answer: True

(d) Total torque is given be

Answer: False, there is no sense in adding torques about 2 different axes.

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