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 swings past the mean position due to the concept of inertia and the conservation of energy. When the bob reaches the mean (equilibrium) position, the net force acting on it does indeed become zero. However, since the bob already has some momentum or inertia, it continues to move forward, exceeding the mean position momentarily.
To understand this better, let's explore the forces acting on a pendulum. When the pendulum is at the extreme point of its swing, the force of gravity acts on it, pulling it back towards the equilibrium position. As it moves towards the mean position, the force of gravity changes direction and starts pulling it back again. At the mean position, the force of gravity is still present, but it acts perpendicular to the direction of motion and does not contribute to the pendulum's forward or backward movement. At this point, the net force is zero.
However, since the pendulum bob has inertia, it maintains its forward velocity even when the net force is zero. This causes it to momentarily swing beyond the mean position. As it moves past the mean position, gravity starts pulling it back, slowing down its forward motion and eventually bringing it to a stop at the opposite extreme point of the swing. This back and forth movement continues, making the pendulum bob swing back and forth.
It's important to note that while the pendulum bob swings past the mean position, the net force acting on it is not zero during this part of the motion. The force of gravity acts on it, providing the necessary acceleration to reverse its direction. The net force becomes zero only when the pendulum bob reaches the extreme points of its swing.
In summary, even though the net force acting on the pendulum bob is zero at the mean position, its inertia causes it to swing past the mean position and continue oscillating due to the force of gravity.