Question

A steel ball of mass 50 g is rolled from the left toward a ball of lead of mass 500 g. The steel ball is traveling with a velocity of 5 m/s. If no external forces act on either of the interacting bodies, then what will be the direction of the momentum of the steel ball after the head-on collision?
A) from left to right as it started off
B) from right to left, making a 180° reversal
C) at an angle of 90° to the initial direction of motion
D) at an angle of 270° to the initial direction of motion

Answer 1

Answer: It's hard to say without characterizing the collision. But it will be either A if the collision is totally in-elastic, or B if the collision is totally elastic. It could be anywhere in between for partially elastic collisions.

Explanation:

momentum is conserved, so initial system momentum will be left to right.

The velocity of the center of mass is 50(5) / 550 = 0.4545... m/s

In an elastic collision, the lead ball will move off at twice that speed or 0.91 m/s to the right.

The steel ball will bounce back and move away at 0.91 - 5 = -4.1 m/s . The negative sign indicates the steel ball has reversed course and has negative momentum

In a totally in-elastic collision, both balls would move to the right at 0.45 m/s. The steel ball will still have positive momentum.