Answer
The average magnitude of the force that has to be applied is $37.3N$
Work Step by Step
1) Find the acceleration $a$ of the fist
Initial velocity $v_0=0$, final velocity $v=8m/s$ and time $t=0.15s$
$$v=v_0+at$$ $$a=\frac{v-v_0}{t}=53.3m/s^2$$
2) To be able to reach $a=53.3m/s^2$, the average magnitude of the force that has to be applied to the fist with $m=0.7kg$ is $$\sum F=ma=37.3N$$
