A circular wooden disc of radius 0.20 m is free to rotate about a vertical axis…
PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A circular wooden disc of radius 0.20 m is free to rotate about a vertical axis passing through its centre O. Two students stand at opposite ends of a diameter and pull on light strings attached tangentially to the rim.
• Student A pulls with a horizontal force of 12 N directed anticlockwise when viewed from above.
• Student B pulls with a horizontal force of 18 N directed clockwise when viewed from above.
Assume the forces remain in the horizontal plane and act at the rim of the disc.
(i) State the physical meaning of torque and write the scalar expression for the magnitude of torque produced by a force that makes an angle θ with the position vector from the chosen axis.
(ii) With reference to this situation, explain the sign convention you would adopt for clockwise and anticlockwise torques about the axis through O.
(iii) Calculate the individual torques produced by the two students about the given axis, indicating their signs clearly.
(iv) Hence find the net torque acting on the disc and state the sense (clockwise/anticlockwise) in which the disc will start rotating.