Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A spool of mass rests on a rough horizontal floor on its two outer rims, of radius . A light string is wound round its axle, of radius . The moment of inertia of the spool about its axis is . The string leaves the bottom of the axle and is pulled horizontally with a steady tension of , as shown in the figure. The spool rolls without slipping.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. For rolling without slipping, take torques about the contact point P: , with , and . Friction, the normal force and the weight have no torque about P. Then Newton's second law gives the friction.
Step 1. String from the bottom of the axle. Its line is above the floor. , so and , in the direction of the pull.
Friction: , so . That is against the pull.
Step 2. String from the top of the axle. Its line is up. , , . Friction: . No friction is needed at all: about the centre, the string's torque is exactly for this rolling.
Step 3. Pulled at an angle. The string leaves the axle along a tangent, so its line is always a distance from the centre. Its torque about P is . It is zero when the line passes through P: , . For smaller angles the spool rolls towards the pull; for larger ones it rolls away from the pull.
Answer: (a) towards the pull; friction against the pull. (b) , with no friction. (c) .
Check. About the centre in (a): the string's torque turns the spool backwards, while friction's torque turns it forwards, and the difference gives : (all in ).
Watch out. Taking torques about the contact point works here because the centre of mass is at the spool's axis, a fixed height above that point. For a body whose centre of mass is off its axis, it would not.
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