If you tie a small spherical lump of clay to the end of a long thread and fix the other end of the thread on the ceiling then you will have constructed a simple pendulum for yourself. A simple pendulum is a wonderful and very simple physical device that finds many uses, like in a clock.
What does a simple pendulum do? If you hold the ball of clay in your hand, pull it towards you and release it, you will find that the ball takes almost an equal amount of time to move from the extreme position back to the center and again to an extreme position at the other end. And the pendulum keeps repeating this motion till it is brought to a halt because of air friction.
When you pull the ball of clay towards you and bring it to one extreme, why does the pendulum seek a return to its original position? The force of gravity or the weight of the pendulum acting on the pendulum is the restoring force that acts on the simple pendulum. Gravity is nothing but the force with which the earth attracts every object towards its center. The other force that acts on the pendulum is the tension in the thread. This force always acts along the direction of the thread. Since the pendulum does not move in the direction dictated by the thread, we can confidently say that tension force is balanced by a component of the force of gravitation acting on the simple pendulum.
Time period of a simple pendulum is the time taken by the ball of clay to go from one extreme end to the other and back to the same extreme end. This time period usually depends on three factors. The first one of them is the length of the simple pendulum. The time period of the simple pendulum is proportional to the length of the pendulum. If you decrease the length of the thread, you will find that the time taken the by ball to go from one extreme end to the other and back also decreases. The second factor affecting the time period is the value of acceleration due to gravity in the place at which the calculations are being made. Third factor is the small angle by which the pendulum was deflected from its center position by you.
Potential to kinetic and back
As you pull the pendulum to one extreme position, its height from the ground increases and thus its potential energy also increases. When you release the pendulum, the ball goes from one extreme position to the center. As its speed increases, the kinetic energy associated with it also increases till it reaches a maximum at the center and potential energy decreases to zero. As the pendulum overshoots and begins to rise again, the speed of the bob decreases naturally. This is because kinetic energy is being converted into potential energy again. This keeps on happening on the way back too. So it is an important device to study theoretical aspects of potential energy to kinetic energy changes.