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Vibrations

Energy in simple harmonic motion

  • 3 things to know
  • Component 1

Overview

Recall questions on the interchange between kinetic and potential energy through the cycle, where each is greatest and where zero, why the total stays constant in the absence of damping, and the shape of the energy against displacement graph.

What you need to know

Answer each one before you reveal it.

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Key term

Describe how the kinetic and potential energies of an undamped simple harmonic oscillator vary through one cycle.

Key term

State the total energy of a simple harmonic oscillator in terms of its mass, angular frequency and amplitude.

Key term

State the total energy of a mass-spring oscillator in terms of the spring stiffness and the amplitude.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe how the kinetic and potential energies of an undamped simple harmonic oscillator vary through one cycle.

    Model answer

    The energy is entirely potential at the two extremes and entirely kinetic at the equilibrium position, and the two interchange continuously while their sum stays constant.

  2. 2

    State the total energy of a simple harmonic oscillator in terms of its mass, angular frequency and amplitude.

    Model answer

    The total energy is E = ½mω²A², where m is the mass in kg, ω the angular frequency in rad s⁻¹ and A the amplitude in m.

  3. 3

    State the total energy of a mass-spring oscillator in terms of the spring stiffness and the amplitude.

    Model answer

    The total energy is E = ½kA², where k is the stiffness in N m⁻¹ and A the amplitude in m.

  4. 4

    Write an expression for the kinetic energy of a simple harmonic oscillator at displacement x.

    Model answer

    The kinetic energy is Eₖ = ½mω²(A² − x²), which follows from ½mv² with v = ω√(A² − x²).

  5. 5

    Write an expression for the potential energy of a simple harmonic oscillator at displacement x.

    Model answer

    The potential energy is Eₚ = ½mω²x². That expression is the total energy minus the kinetic energy at that displacement.

  6. 6

    Describe the shape of a graph of potential energy against displacement for a simple harmonic oscillator.

    Model answer

    The graph is a parabola with its minimum at the origin, since the potential energy is proportional to the square of the displacement.

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