I got stucked with a solution of a problem. I have a pendulum of r radius which is in G gravitational field. So the working acceleration on the bob is a function of the angle. That is Gcosθ. The mass of the bob = m The radius of the string of the pendulum = r Gravitational force is G. So the magnitude of the component of the force that is working on the bob is Gcosθ. My question is what time is needed to reach at the bottom where θ=0 rad. I know the acceleration is a function of time and its integral is velocity and its integral is the position. But here the acceleration is a function of position (or angle). So I can't convert the parameter of the function.