There are two types of problem on impulsive motion which receive a somewhat cursory treatment in most of the standard text books on dynamics. The first, and more elementary, is as follows: Two rods AB and BC are jointed at B and an impulse J is applied to one of them at a certain point. Find the motion. As one of the steps in the solution of this problem we are usually recommended to take moments about the point B. But, we may remark, B is neither a fixed point nor the centre of gravity of the system. How then are we justified in taking moments about this point? This question will be considered in paragraph 2 below.