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?
The pendulum bob, when it reaches the mean position, experiences a zero net force. At this point, the gravitational force acting on the bob is balanced by the tension in the string. However, the pendulum swing does not stop at the mean position because of the concept of inertia.
Inertia is the tendency of an object to resist changes in its motion. When the pendulum bob reaches the mean position, it has acquired a certain amount of kinetic energy due to its initial displacement from the mean position. The bob continues to move past the mean position because of its inertia. It tends to keep moving in the same direction and overcomes the zero net force momentarily.
As the bob moves past the mean position, the tension in the string starts to pull it back towards the equilibrium position. The bob slows down due to the opposing force of gravity, but its inertia keeps it moving. Eventually, the bob reaches the furthest point on the other side, where the tension in the string again balances the gravitational force. The same cycle then repeats in the opposite direction.
In summary, while the net force on the bob is zero at the mean position, its inertia keeps it moving past that point until the balanced forces bring it to a halt before returning in the opposite direction.