Question

A surface is hit elastically and normally by n balls per unit time. All the balls have same mass m and moves with the same velocity u. The force on the surface is :

• A. 1/2 mnu${}^2$
• B. mnu${}^2$
• C. 2 mnu${}^2$
• D. 2 mnu

Answer 1

Answer: (D)

2 mnu

Here, a surface is hit elastically and normally by $n$ balls per unit time. The mass of $n$ balls is $mn$ and velocity of each ball is $u$.
The momentum of balls before collision is $mnu$ and after collision is $-mnu$
Now, the force exerted by the balls on the surface is
$F=\frac{\Delta p}{t}$
$F=\frac{mnu-(-mnu)}{1}=2mnu$