Consider some material object, more or less rigid, with two ends, A and B, like(adsbygoogle = window.adsbygoogle || []).push({});

A---B

It is at rest at a point in time t_0 in my reference frame. Now I kick it a bit, i.e. I apply some force for a limited amount of time at A in the direction of B. After the kick, the whole object has a speed v in the direction A->B. I reckon that the speed of B, v_B, is never larger than that of A, v_A, before, during and after the whole experiment.

Now I consider how, during application of the force, the force propagates through the object from A to B. The speed of propagation is limited by the speed of light c.

As a consequence it seems that during the force application, there is a small time interval where v_B<v_A. Integration of this delta-speed translates into a reduction of the distance between A and B at the end of the experiment.

If I did not make a mistake, the reduction factor is (1-v/c).

This is a stronger reduction even than the relativistic length contraction factor. How can I get the object longer again to match the relativistic contraction? To me it feels wrong to argue with material dynamics to fix an effect that directly results from a very basic principle.

Ideas?

Harald.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Force propagation and length contraction

**Physics Forums | Science Articles, Homework Help, Discussion**