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 down’ from the truck in the 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 $\frac{\Delta m}{\Delta t}$ 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).