# The Naked Scientists Forum

### Author Topic: How rapidly is force transmitted along a cable?  (Read 1782 times)

#### thedoc

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##### How rapidly is force transmitted along a cable?
« on: 29/04/2012 12:33:01 »

If we build a cable let's say 1 million Km long and we stretch it using 2 spaceships and if one of the spaceships pulls, doesn't the other feels the pull instantaneously (faster than light) ?

What do you think?
« Last Edit: 29/04/2012 12:33:01 by _system »

#### yor_on

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##### Re: How rapidly is force transmitted along a cable?
« Reply #1 on: 29/04/2012 13:53:57 »
No, the 'force'' from it has to be transmitted through the cable, usually expressed at about the speed of sound. But if you go to here. you can see how to count on it as it depends on the material, mass per unit length (thickness) and tension in which the force/waves propagates.

#### Phractality

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##### Re: How rapidly is force transmitted along a cable?
« Reply #2 on: 29/04/2012 18:53:48 »
For a long rod or cable, longitudinal pressure waves propagate at the speed given by this formula:

Y is Young's modulus and ρ is the density of the material.

For a typical steel alloy, longitudinal waves propagate about 6 km/s.

Of course, this depends on the rod remaining in tension. If you push one end of the rod hard enough to reduce the tension to zero, the wave becomes transverse, rather than longitudinal, and the speed is far slower.

#### yor_on

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##### Re: How rapidly is force transmitted along a cable?
« Reply #3 on: 01/05/2012 15:44:26 »
apropos forces in a cable.

This one was a real pleasure to read.
The speed of electricity.

#### The Naked Scientists Forum

##### Re: How rapidly is force transmitted along a cable?
« Reply #3 on: 01/05/2012 15:44:26 »