a) If the net external torque on a system of particles about an axis is zero, then
→
τ
=
−
r
×
→
F
= 0 When forces act radially , θ = 0 ⇒ |τ| = 0 b) When forces are acting on the axis of rotation, r = 0,|τ| = 0 c) When forces acting parallel to the axis of rotation θ = 0°,|τ| = 0 d) When torque by forces are equal and opposite, τnet = τ1−τ2 = 0