Question

Inner and outer radii of a spool are r and R, respectively. A thread is wound over its inner surface and spool is placed over a rough horizontal surface. Thread is pulled by a force F as shown in Figure. In case of pure rolling, which of the following statements are possible to be correct

• A. Thread unwinds, spool rotates anticlockwise and friction acts left wards
• B. Thread winds, spool rotate clockwise and friction acts leftwards
• C. Thread winds, spool moves to the right and friction acts rightwards.
• D. Thread winds, spool moves to the right and friction come into existence

Answer 1

Answer: (A), (C), (D)

The correct options are
A Thread unwinds, spool rotates anticlockwise and friction acts left wards
C Thread winds, spool moves to the right and friction acts rightwards.
D Thread winds, spool moves to the right and friction come into existence

If torque is taken about the point of contact, F will develop a clock -wise moment and hence wR will be to left. Hence, friction will be to the right or the thread winds.