A 120 kg mass, initially at rest, is blown apart into a 80 kg piece and a 40 kg piece. After the blast, the two masses are moving apart with a relative velocity of 60 m/s. The speed of the 80 kg mass is
A) 59 m/s.
B) 51 m/s.
C) 34 m/s.
D) 20 m/s.
E) 14 m/s.

Question

Quizplus · Accepted Answer

Using conservation of momentum, the initial momentum is zero. Let v be the velocity of the 80 kg mass and u be the velocity of the 40 kg mass. The relative velocity is given by v - u = 60 m/s. Solving the equations 80v + 40u = 0 and v - u = 60, we find v = 20 m/s.

A 120 Kg Mass, Initially at Rest, Is Blown Apart