Question

A truck carrying sand is moving on a smooth horizontal road with a uniform speed u. If a mass $\Delta m$ of sand leaks in a time $\Delta t$ from the bottom of the truck, the force needed to keep the truck moving at its uniform speed u is given by:

• A. $\frac{\Delta mu}{\Delta t}$
• B. $\frac{\Delta mu}{2\Delta t}$
• C. $\frac{\Delta m{u}^2}{\Delta t}$
• D. $0$

Answer 1

Answer: (D)

$0$

Answer is (D).

On smooth horizontal road there is no friction.

The sand falling from the truck in the vertical downwards direction, exerts force on the truck in the vertically upward direction due to change in momentum. So no force is required to maintain the speed $u$ in horizontal direction.