In general, the vector sum of the external torques equals $\dfrac{\mathrm{d}\mathbf{L}}{\mathrm{d}t}$, the rate of change of the angular momentum vector. If your object is starting at rest, the instantaneous axis of rotation would be in the direction of $\dfrac{\mathrm{d}\mathbf{L}}{\mathrm{d}t}$. Otherwise, it would be in the direction of $\mathbf{L}$, which may be changing with time. Note: a torque relative to the centre of mass is given by $\mathbf{R} \times \mathbf{F}$ (vector product).
R.W. Bird
- 12.2k
- 2
- 9
- 20