A 7000-kg coal car of a train coasts at 7.0 m/s on a frictionless track when a 3000-kg load of coal is dropped vertically onto the car.The coal car's speed after the coal is added is
A)2.1 m/s
B)3.0 m/s
C)4.9 m/s
D)7.0 m/s
E)16 m/s

Question

Quizplus · Accepted Answer

The total momentum before and after the coal is added must be conserved. Initially, the momentum is (7000 kg)(7.0 m/s). After the coal is added, the total mass is 10000 kg. Solving for the final velocity: (7000 kg)(7.0 m/s) = (10000 kg)(v), gives v = 4.9 m/s.

A 7000-Kg Coal Car of a Train Coasts at 7