Question

When a rod moves at a relativistic speed v, its mass

• A. must increase by a factor of $\gamma$
• B. may remain unchanged
• C. may increase by a factor other than $\gamma$
• D. may decrease

Answer 1

The correct option is A must increase by a factor of $\gamma$

Mass of moving object $m=\frac{m_o}{\sqrt{1-\frac{v^2}{c^2}}}=m_o\gamma$ where $m_o$ is the rest mass and $\gamma =\frac{1}{\sqrt{1-v^2/c^2}}$

Thus mass of the moving rod must increases by a factor of $\gamma$.