Why does a porter bend forward while carrying a sack of rice on his back?

Two identical water bottles one empty and the other filled with water are allowed to roll down an inclined plane. Which one of them reaches the bottom first? Explain your answer.

A rectangle block rests on a horizontal table. A horizontal force is applied on the block at a height h above the table to move the block. Does the line of action of the normal force N exerted by the table on the block depend on h?

Three identical solid spheres move down through three inclined planes A, B and C all same dimensions. A is without friction, B is undergoing pure rolling and C is rolling with slipping. Compare the kinetic energies E_A E_B and E_c at the bottom.

An electron of mass 9\(\times\)10^-31 kg revolves around a nucleus in a circular orbit of radius 0.53 \(\overset { 0}{ A } \). What is the angular momentum of the electron? (Velocity of electron is, v = 2.2 \(\times\)10⁶ ms^-1)