A 2.0-kg ball moving eastward at 3.0 m/s suddenly collides with and sticks to a 4.0-kg ball moving northward at 2.0 m/s.What is the magnitude of the momentum of this system just after the collision?
A)14 kg ∙ m/s
B)2.0 kg ∙ m/s
C)10 kg ∙ m/s
D)6.0 kg ∙ m/s
E)8.0 kg ∙ m/s

Question

Quizplus · Accepted Answer

Before the collision, the momentum of the first ball is (2.0 kg)(3.0 m/s) = 6.0 kg·m/s to the east, and the momentum of the second ball is (4.0 kg)(2.0 m/s) = 8.0 kg·m/s to the north. After the collision, the two balls stick together and move in a new direction. By the law of conservation of momentum, the total momentum of the system before the collision must equal the total momentum after the collision. The horizontal component of momentum that the system had before the collision is now the total momentum of the system, since the two balls move together in the horizontal direction. Therefore, the magnitude of the momentum of the system just after the collision is 6.0 kg·m/s, which is choice C.

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