Final lesson

J R Copley is confused about the centre of mass and centre of gravity (Letters, April). As a retired physics teacher, allow me to clarify it. The centre of mass is where the mass of rider and bike is concentrated. The centre of gravity is the point at which the pull of gravity acts. These are coincident points in Earth’s gravity field – so when the rider stands on the pegs, his centre of mass and his centre of gravity both rise.

The rider’s mass does not act on seat or pegs, though his weight (subtly different to mass) does – but these are internal forces with respect to the bike and rider and have no effect on stability. The bike is more stable when you stand on the pegs because of the inverted pendulum effect, which is enhanced when the centre of gravity is raised. Bob Goode