A2 Level · Topic 17.2
A-Level 9702 / Topic 17 / A2

Kinetic and potential, trading places.

Energy is the other way to read an oscillation. It is conserved overall, but it shuttles between kinetic and potential in a way fixed entirely by the amplitude.

The key idea

In SHM kinetic and potential energy interchange while the total stays constant. KE is greatest at the centre, PE at the extremes, and the total is E = ½mω²x₀².

total energy KE PE −x₀ 0 +x₀ energy
Fig. 1 — In SHM energy shuttles between KE (max at the centre) and PE (max at the extremes); their sum is constant
Section 01

The energy swing.

As an oscillator moves, energy sloshes between kinetic and potential. At the centre it is all kinetic, at the extremes all potential, and at every point in between the two add up to the same total. The simulation shows the bars trading height while the total bar stands still.

Section 02

Total energy, fixed by the amplitude.

The total energy of an undamped oscillation is E = ½mω²x₀². The potential energy is ½mω²x² and the kinetic energy is the remainder, ½mω²(x₀² − x²). Because E depends on the square of the amplitude, doubling the amplitude makes the energy four times as large.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks on the energy interchange and the total energy. Each correct answer earns XP and lights this skill on your star map.

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For an undamped oscillator, the total energy during the motion is:

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The kinetic energy of an oscillator is greatest:

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The total energy of an oscillation is E = ½mω²x₀². Doubling the amplitude changes the total energy to:

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At the extremes of the motion (x = ±x₀), the energy of the oscillator is:

Examiner trap

The total energy is proportional to the amplitude squared (and to ω²), so doubling the amplitude quadruples the energy. Kinetic energy is greatest at the centre and potential energy greatest at the extremes; their sum is constant only while there is no damping to drain energy away.

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Energy in simple harmonic motion

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