When the pendulum bob reaches the mean position, the net force acting on it is zero. Why then does it swing past the mean position?
IT has momentum.
The reason a pendulum swings past its mean position, even though the net force acting on it is zero at that point, is due to inertia. Inertia is the tendency of an object to resist changes in its motion. When the pendulum bob reaches the mean position, it possesses potential energy (due to its elevated position) and kinetic energy (due to its velocity). Because of inertia, the bob continues to move in its original direction, even though the net force is momentarily zero.
To understand this concept further, let's break it down step by step:
1. Initial swing: When the pendulum is released from an elevated position, it moves towards its mean (equilibrium) position due to the force of gravity. As it approaches the mean position, the force of gravity is partially converted into kinetic energy, causing the pendulum bob to gain speed.
2. Mean position: At the mean position, the bob momentarily comes to a stop. It is at this point that the net force acting on the bob is zero. The force of gravity pulling the bob downwards is balanced by an equal and opposite force exerted by the pivot or string, resulting in a net force of zero.
3. Inertia takes over: Even though the net force is zero, the bob possesses both potential energy (due to its height above the mean position) and kinetic energy (due to its velocity). According to Newton's first law of motion, an object in motion will continue moving in a straight line with constant speed unless acted upon by an external force. This principle is known as inertia.
4. Swing past the mean position: As a result of inertia, the bob continues to move in the same direction it was already moving before reaching the mean position. This causes the pendulum to swing past the equilibrium position, gradually losing height and converting its kinetic energy back into potential energy.
5. Oscillation: The pendulum continues swinging, moving back and forth between its maximum amplitude positions on either side of the mean position. The cycle of swinging back and forth is known as oscillation, and it continues until external forces such as air resistance or friction eventually dampen the motion.
In summary, while the net force is momentarily zero at the mean position of the pendulum, the bob's inertia keeps it moving past that point. This behavior is due to the conservation of energy and Newton's first law of motion.