Q. Explain why one leans forward while climbing a hill.
A. The person leans forward in order to keep himself in stable equilibrium. By leaning forward, he increases the base of the support, so that the vertical line passing through his centre of gravity may fall within the base. For similar reasons, a man has to bend backward while climbing down a hill.
Q. Why is it dangerous to allow extra passengers on the upper deck of a double-decker bus?
Explain why passengers in a boat are not allowed to stand.
A. This is done so that the centre of gravity of the bus is not raised and the bus may not topple over due to unstable equilibrium. For the similar reason, passengers in a boat are not permitted to stand.