(a) As the body is moving with uniform speed after the action of three forces, \(\vec{F_1}\), \(\vec{F_2}\) and \(\vec{F_3}\) on a point on body.
Since the body is mobbing with no acceleration, the sum of the forces is zero F1 + F2 + F3 = 0. Let F1, F2, F3 be the three forces passing through a point. Let F1 and F2 be in the place A (one can always draw a plane having two intersecting lines such that the two line lie on the plane). Then F1 + F2 must in the plane A. Since F3 = -(F1 + F2). F3 is also in the plane A.
(b) Consider the torque of the forces about P. Since all the forces pass through P, the torque is zero. Now consider torque about another point O. Then torque about O is
Torque = OP × (F1 + F2 + F3)
As a resultant of \(\vec{F_1},\vec{F_2}\) and \(\vec{F_3}\) is zero,
Since F1 + F2 + F3 = 0,
Torque = r × \(\vec{F}\) = 0.