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How does a pendulum change between kinetic energy and potential energy?
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mechanics
Figure 1: (A) The vector sum C = A + B = B + A. (B) The vector difference A + (−B) = A − B = D. (C, left) A cos θ is the component of A along B and (right) B cos θ is the component of B along A. (D, left) The right-hand rule used to find the direction of E = A × B and (right) the right-hand rule used to find the direction of −E = B × A.
In a swinging pendulum, mechanical energy—the sum of kinetic energy and potential energy—is continuously exchanged as the pendulum moves.12
- At the extremities of the swing: The pendulum reaches its maximum height, meaning its potential energy is at its greatest. At these points, the pendulum stops briefly, so its speed is zero and its kinetic energy is at its minimum.12
- At the vertical (equilibrium) position: The pendulum reaches its lowest point, resulting in its least potential energy. Because the pendulum is moving at its maximum speed here, its kinetic energy is at its greatest.1
As the pendulum swings upward, kinetic energy is converted into potential energy; as it swings back down, potential energy is converted back into kinetic energy. In an idealized system without friction or air resistance, the total mechanical energy remains constant. In reality, dissipative forces gradually convert this energy into thermal energy, causing the swing amplitude to decrease over time.213