Drag the pendulum bob and release it. Watch PE, KE, thermal energy, and total energy.
Gravitational potential energy belongs to the Earth + pendulum bob system, not the bob alone.
With friction, mechanical energy decreases, but that energy is transferred into thermal energy by non-conservative work.
Total energy is conserved for the larger system.
This interactive 3D physics simulation demonstrates the law of conservation of energy using a pendulum. Students can explore the transformation between gravitational potential energy, kinetic energy, and thermal energy.
A key idea in energy conservation is the definition of the system. For a pendulum, gravitational potential energy is not a property of the bob alone. It belongs to the interaction between the Earth and the pendulum bob. Therefore, the gravitational potential energy is defined for the Earth + bob system.
When friction is zero, the pendulum continuously transforms energy between potential energy and kinetic energy. At the highest point, potential energy is maximum and kinetic energy is minimum. At the lowest point, kinetic energy is maximum and potential energy is minimum.
When friction or air resistance is present, the mechanical energy of the pendulum decreases. However, the energy does not disappear. It is transferred to thermal energy by non-conservative work. For the larger system including the surroundings, the total energy remains conserved.
Gravitational potential energy is associated with the Earth-bob interaction, not only the bob by itself.
As the pendulum swings downward, potential energy decreases and kinetic energy increases. As it swings upward, kinetic energy decreases and potential energy increases.
Friction reduces mechanical energy, but the lost mechanical energy appears as thermal energy. This helps students understand non-conservative work.
For the larger system including the Earth, bob, and surroundings, total energy remains constant even when mechanical energy decreases.