Answer:
6.85 m/s
Step-by-step explanation:
We can solve the problem by using the law of conservation of momentum.
In fact, since there are no external forces acting, the total momentum before and after must be conserved. So we can write:

where
is the initial mass of the car
is the initial speed of the car
is the mass of the car after the load of gravel is dropped
v2 is the final speed of the car
Solving for v2, we find
