torque (moment of force or couple) A torque acting on a rigid body about a fixed point produces angular acceleration. The magnitude and direction of the torque vector, t is given by the vector product,

t = r´ F = r´ F sin q n (t6)

where r is the radius vector from the fixed point to the point of application of the force, F (see fig. t8). q is the smaller angle between r and F. n is a unit vector perpendicular to the plane containing the vectors r and F and given by the right hand screw rule (see under ‘vector product’).

If L denotes the angular momentum * then the torque is also given by,

t = dL/dt (t7)

For a rigid body with moment of inertia I, the torque is given by,

t = I a (t8)

where a is the angular acceleration *.