[SOLVED] Finding Velocity for Work and Energy Problem A 2 kg physics textbook is dropped form rest out a window 10 meters above the ground. What is the textbook's speed when it's kinetic and potential energies are equal? The answer is v = 9.90 m/s. You would use the equation U = (1/2)kx^2 and K = (1/2)mv^2, right? I really don't know what to do especially since the k is in the equation of U. How do I solve this?