Question

One skateboarder has a mass of 80 kilograms and moves at a speed of 8 meters per second. A second skateboarder has a mass of 60 kilograms. How fast must the second skateboarder move to have the same kinetic energy as the first one?

Answer 1

Set their kinetic energies equal:
(1/2) m1 v1^2 = (1/2) m2 v2^2 ⇒ m1 v1^2 = m2 v2^2

So v2 = v1 sqrt(m1/m2) = 8·sqrt(80/60) = 8·sqrt(4/3) = 16/√3 ≈ 9.24 m/s.