The Force Driving the Swing: Gravity
A pendulum swings primarily due to the force of gravity. When a pendulum bob is pulled to one side and released, gravity acts to pull it downwards towards its lowest equilibrium position. This gravitational pull is the restoring force that initiates and perpetuates the back-and-forth motion.
Continuous Energy Transformation
As the pendulum swings, there's a constant conversion between two forms of energy: gravitational potential energy and kinetic energy. At the highest points of its swing, the pendulum has maximum potential energy and momentary zero kinetic energy. As it falls, potential energy converts to kinetic energy, reaching maximum kinetic energy (and speed) at the lowest point of its arc. This kinetic energy then carries it upwards against gravity to the other side.
Inertia and the Fixed Pivot
The pendulum bob's inertia, its tendency to resist changes in motion, keeps it moving through the lowest point of its swing. Simultaneously, the fixed pivot point constrains the bob's movement to an arc. This combination means the bob cannot simply drop but must follow a curved path, causing it to rise on the opposite side until its kinetic energy is fully converted back into potential energy.
Sustained vs. Damped Oscillation
In an ideal scenario, without any energy loss, a pendulum would swing indefinitely. However, in reality, factors like air resistance (drag) and friction at the pivot constantly remove small amounts of mechanical energy from the system. This damping causes the amplitude of the swing to gradually decrease until the pendulum eventually comes to rest at its equilibrium position.