A small object of mass m is projected vertically upwards. Its rate of change of momentum $\frac {dp_y}{dt}$ at some instant as it moves upwards ignoring air resistance is given by,

(A) mg
(B) -mg
(C) 0
(D) 2mg

Solution

The only force acting = mg

Rate of change of momentum $\frac {dp_y}{dt} = -mg$ as the object is moving upward whereas the force mg is downwards.

Hence, (B)