A student is analysing the rotation of a uniform disc that can freely turn about a…
PHYSICS · Multi-part · CBSE · Physics · System of Particles and Rotational Motion · System of Particles and Rotational Motion
Problem
A student is analysing the rotation of a uniform disc that can freely turn about a fixed axis perpendicular to its plane and passing through its centre. The disc is acted upon by several forces, and the sign of each torque must be assigned before proceeding with any calculations.
Answer the following questions related to the sign convention for torque about the central axis of the disc.
(i) Explain the criterion generally used in textbooks to decide whether the scalar value of a torque about an axis is taken as positive or negative.
(ii) If the student initially chooses counter-clockwise rotation as positive, describe qualitatively how the signs of all torques will change if the student subsequently chooses clockwise rotation as positive instead.
(iii) Two equal and opposite forces act at equal distances on opposite sides of the axis so that one tends to turn the disc clockwise while the other tends to turn it counter-clockwise. State the sign of each torque for each of the two sign conventions mentioned above and justify your answer without performing any numerical calculation.