# A famous relation in physics relates 'moving mass' m to 'rest mass' $m_0$ of a particle in terms of its speed v and the speed of light c. (This relation first arose as a consequence of special theory of relativity due to Albert Einstein). A boy recalls the relationship almost correctly but forgets where to put the constant c. Guess where to put the missing c.

$m = m_0 \bigg( 1- \large\frac{v^2}{c^2} \bigg)^{-\large\frac{1}{2}}$