A 5000 kg open train car is rolling on frictionless rails at 22.0 m/s when it starts pouring rain. A few minutes later, the car’s speed is 20.0m/s.

What mass of water has collected in the car?

Known Variables:

Mass of train (m1) = 5000 kg
v1 = 22 m/s
v2 = 20 m/s
Conservation of momentum:
Pi – Pf = 0
m1  x  v1 = (m1+m_water)  x  v2
m_water = (m1 * v1 / v2) – m1

m_water = [5000 x (22/20)] – 5000

Mass of water collected:
m_water = 500 kg