Why are two ends of a long bridge generally kept on trolley?

Why compounds of Zn, Cd, Hg are generally white ?

Why transition metals are generally coloured ?

Why are leaves generally green ?

If we place our ar at the end of a long pipe and ask our friend to hammer at the other end,we will hear two distinct type of sound.Explain why?

In rabbit, end of a long bone is connected to another bone by

Why is the pyramid of biomass in sea generally inverted ?

A child sits stationary at one end of a long trolley moving uniformly with a speed V on a smooth horizontal floor. If the child gets up and runs about on the trolley in any manner, what is the speed of the CM of the (trolley + child) system ?