Question

$n$ small balls, each of mass $m$, impinge elastically each second on a surface with velocity $u$. The force experienced by the surface will be

• A. $mnu$
• B. $2mnu$
• C. $4mnu$
• D. $\frac{1}{2}mnu$

Answer 1

The correct option is B

$2mnu$

Since the collision is elastic, the balls rebound with the same speed. Therefore, the change in momentum of each ball = 2mu.
The change in momentum per second due to n balls = 2mnu. But the change in momentum per second is the force. Hence, the correct choice is (b).