Discussion
A ball of 200 g mass moving with a speed of 5 ms-1collides with a wall and bounces back with the same speed. If the force exerted on the wall is 1 N, then the ball is in contact with the wall for
Answer: A
The contact time is 2 s.
The ball reverses with the same speed, so its momentum changes from \(+mv\) to \(-mv\):
\[ \begin{aligned} \Delta p &= 2mv = 2(0.2)(5) = 2\ \text{N s} \\ t &= \frac{\Delta p}{F} = \frac{2}{1} = 2\ \text{s} \end{aligned} \]
1 s is the answer you get by taking the change in momentum as \(mv\), forgetting that the ball bounces back. 0.5 s halves it again.
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