## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

$\frac{v_{son}}{v_{mother}} = 2$
Let $m$ be the mass of the son. Let $4m$ be the mass of the mother. We can equate the kinetic energy of the son and the kinetic energy of the mother to find the ratio of their speeds. $KE_{son} = KE_{mother}$ $\frac{1}{2}mv_{son}^2 = \frac{1}{2}(4m)v_{mother}^2$ $\frac{v_{son}^2}{v_{mother}^2} = 4$ $\frac{v_{son}}{v_{mother}} = 2$