## 84. Math Puzzle $5 / 72$

A lorry with a mass of $800 \mathrm{~kg}$, traveling at a speed of $1.5 \frac{\mathrm{m}}{\mathrm{s}}$, is hit by 600 $\mathrm{kg}$ of gravel falling vertically.

By what amount does the speed of the lorry decrease as a result?
With $m_{1}$ as the mass of the lorry, $m_{2}$ as the mass of the ballast, and $u_{1}$ as the initial velocity, the final velocity is given by

$$
v=\frac{m_{1} u_{1}}{m_{1}+m_{2}} \approx 0.86 \frac{\mathrm{m}}{\mathrm{s}}
$$