Length Contraction: What Does it Mean for Mass?

[ally1h]

Homework Statement

A spear moves past you at a very high speed. As it passes, you measure its length at one half its normal length. From this measurement, what can you conclude about the moving spear's mass?

Homework Equations

The Attempt at a Solution

To have the spear appear that it is 1/2 its length means it is moving at a substantial fraction of the speed of light. And as the spear's velocity approaches the speed of light the relativistic mass appears greater and greater...

But is there some quantitative value I should know? Such as... the mass of the spear is twice its normal mass? Or some equation that relates the relativistic mass to length?

 

[LowlyPion]

I think in special relativity that you generally have the relationship that

Δx' = Δx/γ

Where γ is the Lorentz factor

γ = 1/(1-(v/c)²)^1/2

This can then relate to the mass as

m' = γm

Here is a jumping off page in Wikipedia that might be helpful:
http://en.wikipedia.org/wiki/Special_relativity#Time_dilation_and_length_contraction