I need help to take the time derivative of a physics equation. The equation: ME=1/2 mr^2 (dθ/dt)^2 +mgr θ^2/2 Where ME is the mechanical energy , m is the mass of the object, r is the radius of the path of the object and θ is the angle around the path. Walter Lewin at MIT differentiates it and gets the result: mr^2 (dθ/dt) (d^2 θ/dt^2)+mgr θ dθ/dt Can someone please explain why this is so. I can't understand how that would be the derivative of the function. Please answer.