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The equation in the image made me think of the density case in special theory of relativity !

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    $\begingroup$ It would be helpful if you could actually write a question rather than just posting an equation. $\endgroup$ – ZeroTheHero Feb 7 '19 at 22:56
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Sure it does. If the stick has a linear density $\lambda=m/x$ where x is the length, then if it's moving at velocity $v$ away from you, the length would appear to be:

$$x'=\frac{x}{\gamma}$$

where:

$$\gamma=\frac{1}{\sqrt{1-(v/c)^2}}$$

the, the density measured by the observer would be:

$$\lambda'=\lambda\gamma$$

And because $\gamma>1$, it would appear to be more dense.

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