Question

A railway flat car is rushing along a level frictionless track at a speed of 45m/s. Mounted on the car and aimed forward is a cannon that fires 65 kg cannon balls with a muzzle speed of 625m/s. The total mass of the car, the cannon, and the large supply of cannon balls on the car is 3500 kg. How many cannon balls must be fired to bring the car as close to rest as possible?

Answer 1

4 balls will be fired.
m1 = 3500, v1 = 45
m2 = 625, v2 = ?
apply "m1.v1=m2.v2"
answer comes about 3.8

Answer 2

is the speed of the cannon ball relative to the car or the ground??