from Cavendish Press Ann Arbor

To lose weight, slow down

Just kidding. But actually, it's true.

It is true that your mass (and your weight along with it) increases when you run. The reason people don't notice it is that it increases so very little.

As with most relativistic effects, they are most noticeable at speeds close to the speed of light. The muons (see "length") that are produced 10 miles up by cosmic rays shoot off at speeds of about 99% of the speed of light. The mass of these muons increases to about 7 times their "rest mass." That means they are 7 times more sluggish, and gravity acts on them with 7 times as much force, as when they are moving slowly (as they do in the lab).

If you travelled that fast, someone standing still and trying to push you to go faster would find that, if your normal weight is 140 pounds, you now weigh a thousand pounds.

Here are the results of some calculations that you will learn how to do if you read this chapter. You will notice that all types of energy count, not just the energy of motion.

• A baseball that weighs ½pound (exactly 0.500000000 pound), if pitched at a speed of 90mi/hr, will weigh 0.5000000000000045 pounds.
• If you weigh 140 pounds while running at 4mi/hr, you could lose 0.000000000000039 ounce just by stopping (as weighed by observers who were standing still both while you were walking and after you stopped).
• A proton having been accelerated to a speed of 99.99% of the speed of light by superconducting magnets in a collider will weigh 71 times as much and require 71 times the force as at the beginning to produce the same acceleration.
• A spring that weighs 1.00000000 ounce, if stretched 2 inches by a force of 10 pounds, will weigh 1.00000000000000045oz. due to the stretching energy added to it.
• A glass of water, if heated to the boiling point (but not boiled) gains 0.00000000000187oz due to the heat energy.

Speed limit is related to the fact that, as things go faster and faster, their kinetic energy makes their mass go up, and so they become more sluggish. It becomes harder and harder to increase their speed.

You can see from the numbers above for the muons and the protons, that as the speed of objects comes very close to the speed of light, it becomes increasingly difficult to give them any extra speed.

In the book you will see why this produces a cutoff at the speed of light, 186,000 miles/second. Nothing can go faster than that. Not for reasons of technology, but they become so sluggish that it becomes impossible to force them past that speed barrier.

Onward to Chap 11: Mass-Energy Equivalence
Back to Chapters 6-14 The Phenomena of Relativity

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