Question

A ball moving with a velocity v hits a massive wall moving towards the ball with a velocity u. An elastic impact lasts for a time Dt.

• A. The average elastic force acting on the ball is$\frac{m(u + v)}{\Delta t}$
• B. The average elastic force acting on the ball is$\frac{2m(u + v)}{\Delta t}$
• C. The K.E. of the ball increases by mu (u + v)
• D. The K.E. of the ball remains the same after the collision.

Answer 1

Answer: (B)

The average elastic force acting on the ball is$\frac{2m(u + v)}{\Delta t}$

Answer 2

$\overline{F} = \frac{m\Delta\overline{v}}{\Delta t} = \frac{m\lbrack{\overline{v}}_{f}–{\overline{v}}_{i}\rbrack}{\Delta t}$

$F = \frac{m\left\lbrack (v + 2u)–(–u) \right\rbrack}{\Delta t} = \frac{2m(u + v)}{\Delta t}$