Q.In the Fosbury-flop technique, a high jumper arches the body over the bar so that the body's own centre of mass can actually pass just below the bar while every part of the body itself clears it. Explain, in terms of how the centre of mass of a system is defined, why this is possible for a body of irregular, curved shape.

The centre of mass of a system is a purely mathematical, mass-weighted average position -- it is not required to lie on any actual piece of the body's material (Exercise 1 makes exactly this same point for a uniform ring). By arching sharply backward over the bar, the jumper's body forms a curved, roughly C-shaped arrangement of mass, with the head, shoulders and legs all above the bar but the unoccupied 'hollow' of the curve dipping down below bar height, directly beneath the arched back. …

Apply the same reasoning as for a hollow ring: a curved, non-convex mass distribution can have its centre of mass located in the …

• Assuming the centre of mass of ANY body must lie somewhere on that body's own material -- this is only guaranteed for solid, convex shapes, not curved or hollow ones. …