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Name: Unknown
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002

I was playing with a little desk toy with suspended ball bearings and wondered what would happen if the balls were made of a material that was infinitely rigid. With any real object (like metal) isn't momentum transferred to the opposite ball at the speed of sound of the material? Would momentum be transferred instantaneously with "rigid" balls?

Momentum is not transferred at the speed of sound. The speed of sound is a velocity (meters/second), not a rate of transfer of momentum (kilogram.meters/second) per second. The rate at which momentum is transferred from one object to the other equals the force between the objects: how hard they push on each other.

Still, you are on the right track. Infinitely rigid balls would not compress at all. As the balls meet, their motions would have to be reversed instantaneously or the balls would have to "overlap". This requires an infinite force and an infinite rate of momentum transfer, except for one fact. Infinitely rigid does not mean infinitely strong, only impossible to compress. For an infinite force to exist, both objects must be able to endure an infinite force. When an object is too rigid, this causing a force greater than it can endure, the object shatters. Unless the rigid objects were truly infinitely strong, they would break. If they were both infinitely strong, they would bounce off each other as other objects do. The motions would be the same, but the time actually in contact would be too short to measure.

If one were much stronger than the other, you would have an "armor piercing shell" striking a tank. The shell gives much less than the tank armor. If the shell has enough momentum, the armor gives enough to allow the shell to penetrate.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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