14. A mass of m = 0.5 kg is attached to an unstretched spring with a spring constant of 400 N/m. The mass is pulled a distance x = 15 cm across a frictionless surface and released. What is the earliest time at which the kinetic energy is of the total mechanical energy? 15. A simple pendulum on Mars is found to have a period of 2 seconds. Assuming that the mass of Mars is one tenth the mass of Earth and that the radius of Mars is one-half that of Earth, what is the length of the pendulum?