Question

A truck moving on a smooth horizontal surface with a uniform speed u is carrying stone-dust. If a mass $\Delta m$ of the stone-dust leaks from the truck in a time $\Delta t$, the force needed to keep the truck moving at its uniform speed is

• A. $u\frac{\Delta m}{\Delta t}$
• B. $\Delta m\frac{du}{dt}$
• C. $u\frac{\Delta m}{\Delta t}+\left(\Delta m\right)\frac{du}{dt}$
• D. zero

Answer 1

The correct option is D

zero

The force exerted on the truck due to leaking stone dust is $u\frac{\Delta m}{\Delta t}$ in the vertically upward direction. Since there is no friction, no force is needed to keep the truck moving with a constant speed in the horizontal direction. Hence, the correct choice is (d).