# A-level Physics (Advancing Physics)/Forces and Impulse in Collisions/Worked Solutions

1. Escape velocity from the Earth is 11.2 km−1. How much impulse must be exerted on a 47000 kg payload to get it to travel away from the Earth?

${\displaystyle I=m(v-u)=47000(11200-0)=526.4{\mbox{ MNs}}}$

2. Two billiard balls, of mass 10g, collide. One is moving at 5ms−1, and the other at 2ms−1. After the collision, the first billiard ball is moving backwards at 4ms−1. The collision takes 1 ms. What force was exerted on this ball?

${\displaystyle I=m(v-u)=0.01(5-(-4))=0.09{\mbox{ Ns}}}$

${\displaystyle F={\frac {I}{\Delta t}}={\frac {0.09}{0.001}}=90{\mbox{ N}}}$

3. What impulse and force were exerted on the second ball?

The impulse was -0.09Ns and the force was -90N.

4. A 60 kg spacewalker uses a jet of gas to exert an impulse of 10Ns. How many times would he have to do this to reach a speed of 1 ms−1 from stationary?

${\displaystyle \Delta v={\frac {I}{m}}={\frac {10}{60}}={\frac {1}{6}}{\mbox{ ms}}^{-1}}$

So, the spacewalker would have to do this 6 times to reach a speed of 1ms−1.

5. A 5 kg bowling ball collides with a stationary tennis ball of mass 0.1 kg at 3ms−1, slowing to 2.5ms−1. It exerts a force of 100N on the ball. How long did the collision take?

${\displaystyle F={\frac {m(v-u)}{\Delta t}}}$

${\displaystyle \Delta t={\frac {m(v-u)}{F}}={\frac {5(3-2.5)}{100}}=0.025{\mbox{ s}}}$