Answer
The scale reading exceeds the true weight by $344.3N$
Work Step by Step
a) First, we look at the period when the sand falls from high above. The speed the sand hits the truck is $v_f$, which we will find.
We have $v_0=0, g=9.8m/s^2$ and $s=2m$ $$v_f^2=v_0^2+2gs=39.2$$ $$v_f=6.26m/s$$
b) We assume the sand comes to rest immediately as it hits the truck. According to the impulse-momentum theorem, as the sand hits the truck, the magnitude of net average force acted on the sand is $$\sum\overline F=\Big|\frac{m}{\Delta t}(v_f-v_0)\Big|$$
We have $v_0=6.26m/s$, $v_f=0$ and $m/\Delta t=55kg/s$ $$\sum\overline F=344.3N$$
We call the force acting on the truck by the sand $P$. According to Newton's 3rd law, $P=\sum\overline F=344.3N$
We have the reading on the scale $R$, which equals the truck and sand's true weight plus the force acted on the truck $P$: $$R=W_{true}+P$$
So the scale reading exceeds the true weight by $344.3N$
