100 balls each of mass $$m$$ moving with speed $$v$$ simultaneously strike a wall normally and reflected back with same speed, in time $$t$$ s. The total force exerted by the balls on the wall is

Each ball has mass $$m$$ and moves with speed $$v$$. After striking the wall normally, it bounces back with the same speed.

Change in momentum for each ball:

$$ \Delta p = mv - (-mv) = 2mv $$

Total change in momentum for 100 balls:

$$ \Delta p_{\text{total}} = 100 \times 2mv = 200mv $$

The total force exerted on the wall (by Newton's second law):

$$ F = \frac{\Delta p_{\text{total}}}{t} = \frac{200mv}{t} $$