An open box-car has a mass of 30,000 kg and is moving on a level railroad track at 4.00 m/s.1,000 kg of water falls straight down into the box-car.The velocity of the box-car after the water falls into the box-car is
A) 7.01 m/s.
B) 6.55 m/s.
C) 5.02 m/s.
D) 3.87 m/s.
E) 2.89 m/s.

Question

Quizplus · Accepted Answer

The momentum before and after the water falls must be conserved. Initial momentum = (30000 kg)(4.00 m/s) = 120000 kg·m/s. After the water falls, the total mass is 31000 kg. Let v be the final velocity. So, 31000 kg * v = 120000 kg·m/s. Solving for v gives v = 120000 / 31000 = 3.87 m/s.

An Open Box-Car Has a Mass of 30,000 Kg and Is