Experiment 9: Mr. Einstein drops his wife's favorite potted plant while traveling at a high velocity. It shatters. Meanwhile, his wife is standing in the field watching.

Question: What will they each observe?

Answer: They both agree that the pot shatters when it hits the surface and that Mr. Einstein is in BIG trouble. However, from Mrs. Einstein's point of view, Mr. Einstein's time slows down and she sees the plant fall in slow motion. This raises the question of why the pot shatters if it seems to gently float to the surface of Mr. Einstein's railroad car. The answer has to do with momentum. Momentum is the amount of "punch" something has. Physicists define momentum as the product of mass and velocity (momentum = mass x velocity). For instance, a car hitting you at 1 mph will do a lot more damage than a feather hitting you at 1 mph because it has more "punch". This extra punch or momentum is a consequence of the car's greater mass. Likewise, if both Mr. and Mrs. Einstein agree that the dropped pot shatters, then this is because the momentum of the pot has remained unchanged. However, since Mrs. Einstein sees the pot falling with less velocity, this also means that its mass must have increased in order for momentum to be conserved.

Conclusion: When an object goes fast with respect to us, not only do we see its clocks slow down and its length shorten, we also see an increase in its mass!