# GRE Physics GR 1777 Problem Solution¶

## 034. Relativity (Length Contraction)¶

### You should know¶

$$\large L = L_0 \sqrt{1-(v/c)^2}$$

### Solution¶

A rod measures 1.00 m in its rest system. To make it 0.80 m, an observer must move parallel to the rod at a speed of

$$\large 0.8 = 1.0 \sqrt{1 - (v/c)^2}$$$$\large 1 - \left( \frac{v}{c} \right)^2 = \left( \frac{4}{5} \right)^2$$$$\large \therefore v = 0.60c$$

(B) 0.60$c$