What is the turning force applied to an object rotating about a fixed axis?

• A. Velocity
• B. Mass
• C. Torque
• D. Friction

Answer: (C)

Turning force applied to an object rotating about an axis is torque. Torque is the moment of force that causes or tends to cause rotational motion around that axis. It depends on how far from the axis you apply the force (the lever arm) and the direction of the force relative to that lever arm. The basic relation is τ = r × F, and its magnitude is τ = r F sin(θ). It is greatest when the force is applied perpendicular to the lever arm, giving τ = rF. Torque connects to angular acceleration through τ = I α, where I is the moment of inertia. Velocity is the rate of rotation, not the cause; mass is inertia resisting changes in motion; friction is a resistive force (it can create a torque, but it’s not the turning force itself).