Answer
(a) $\sum F=100.4N$
(b) $\sum F=38.2N$
Work Step by Step
$m_{skier}=73kg$
(a) Find the acceleration of the skier:
We have $v_0=0$, $v=11m/s$ and $\Delta t=8s$. Thus, $$a=\frac{\Delta v}{\Delta t}=\frac{11}{8}=1.375m/s^2$$
The magnitude of the net force acting on the skier is $$\sum F=m_{skier}a=100.4N$$
(b) Find the deceleration of the skier:
We have $v_0=11m/s$, $v=0m/s$ and $\Delta t=21s$. Thus, $$a=\frac{\Delta v}{\Delta t}=\frac{-11}{21}=-0.52m/s^2$$
Here, we are interested only in the magnitude of $a$, so we take $a=0.52m/s^2$.
The magnitude of the net force acting on the skier is $$\sum F=m_{skier}a=38.2N$$
