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So What Is Weight Really?

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Posted by Aurino Souza on April 14, 2003 13:07:34 UTC

Hi Tim,

A couple quick comments:

" there would be no way to measure the weight of a free falling object since the device you would be using to measure that mass would also be falling free along with it. objects fall at the same rate regardless of the value of thier mass "

That is correct, but there is a subtle issue here. If an object that is, say, a mile about the earth's surface has no weight, why is it that it suddenly becomes "heavy" when it touches the surface? The important issue here is that "weight" is not caused by the earth's gravitational field, for the field is just as strong one mile up the atmosphere as it is here on the surface. It has to be something else.

" acceleration is just the rate of change of velocity with respect to time, in short it is only motion changing with respect to time. motion is relative hence acceleration is relative so any free falling object can in fact be considered as not accelerating at all "

That is not correct; acceleration is not relative. According to modern physics, acceleration is absolute since it can be measured from its own frame of reference. If you are inside a rocket in space moving at constant speed you can't feel your movement; but if you suddenly ignite the engines, you will experience that "clutch to the seat" feeling, just like you do when you step on the gas pedal in your car.

I believe there are problems with that story, but that's how it's told to students of relativity.

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