Relativity and Weight

a train is trying to reach the speed of light its length decreases and its mass increases does its weight increase? my son 27 years old says weight does not increase I say it does please answer.
- robert perkins (age 77)
bellflower,ca l.a.

You're right. Your son is saying that only the rest mass counts as part of the weight, not the full inertial mass, which grows with speed. That can't be right. Think of a light ray. Its rest mass is zero. Yet it bends in a gravitational field, as was first measured in 1919. So it can't just be the rest mass that's affected by gravity. It just goes to show that kids these days don't have the proper respect.

Mike W.

(published on 01/25/2012)