# Lorentz contraction

• Swankie

#### Swankie

Hi all, modern physics student here. If the Lorentz contraction occurs at relativistic speeds, how does it gain mass yet get shorter?

The best I can think of is that it has to depend in which reference frame one is in at the time. Does anyone know any useful links where I can read about this?

You're right about the dependence on a reference frame. This forum is a good place to find information on this subject. Do a search for "mass" in the title and pick the threads that address your question. Click on this link to see the result of this search:

https://www.physicsforums.com/search.php?searchid=3078794 [Broken]

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If the Lorentz contraction occurs at relativistic speeds,

Length contraction occurs at ALL non-zero speeds.

how does it gain mass

An object does NOT gain mass due to velocity. It gains "relativistic mass," but relativistic mass is a virtually useless quantity which physicists (for that reason) don't use anymore.

yet get shorter?

Why does this seem paradoxical to you? Why do you think that the measured length of an object would have any effect on its mass?

The best I can think of is that it has to depend in which reference frame one is in at the time.

Well yes, length and relativistic mass are certainly frame-dependent quantities. You could, for example, measure an object from a reference frame in which it is at rest. You could also measure the same object from a reference frame in which it has a velocity of 99.999% the speed of light. Obviously the two measurements will differ.

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