CIE A Level Physics (9702) exams from 2022

Revision Notes

17.1.6 Energy in SHM

Kinetic & Potential Energies

  • During simple harmonic motion, energy is constantly exchanged between two forms: kinetic and potential
  • The potential energy could be in the form of:
    • Gravitational potential energy (for a pendulum)
    • Elastic potential energy (for a horizontal mass on a spring)
    • Or both (for a vertical mass on a spring)
  • Speed v is at a maximum when displacement x = 0, so:

The kinetic energy is at a maximum when the displacement x = 0 (equilibrium position)

  • Therefore, the kinetic energy is 0 at maximum displacement x = x0, so:

The potential energy is at a maximum when the displacement (both positive and negative) is at a maximum x = x0 (amplitude)

  • A simple harmonic system is therefore constantly converting between kinetic and potential energy
    • When one increases, the other decreases and vice versa, therefore:

The total energy of a simple harmonic system always remains constant and is equal to the sum of the kinetic and potential energies

  • The key features of the energy-time graph:
    • Both the kinetic and potential energies are represented by periodic functions (sine or cosine) which are varying in opposite directions to one another
    • When the potential energy is 0, the kinetic energy is at its maximum point and vice versa
    • The total energy is represented by a horizontal straight line directly above the curves at the maximum value of both the kinetic or potential energy
    • Energy is always positive so there are no negative values on the y axis
  • Note: kinetic and potential energy go through two complete cycles during one period of oscillation
  • This is because one complete oscillation reaches the maximum displacement twice (positive and negative)
  • The key features of the energy-displacement graph:
    • Displacement is a vector, so, the graph has both positive and negative x values
    • The potential energy is always at a maximum at the amplitude positions x0 and 0 at the equilibrium position (x = 0)
    • This is represented by a ‘U’ shaped curve
    • The kinetic energy is the opposite: it is 0 at the amplitude positions x0 and maximum at the equilibrium position x = 0
    • This is represented by a ‘n’ shaped curve
    • The total energy is represented by a horizontal straight line above the curves

Exam Tip

You may be expected to draw as well as interpret energy graphs against time or displacement in exam questions. Make sure the sketches of the curves are as even as possible and use a ruler to draw straight lines, for example, to represent the total energy.

Calculating Total Energy of a Simple Harmonic System

  • The total energy of system undergoing simple harmonic motion is defined by:

Calculating Total Energy of a Simple Harmonic System equation 1

  • Where:
    • E = total energy of a simple harmonic system (J)
    • m = mass of the oscillator (kg)
    • ⍵ = angular frequency (rad s-1)
    • x0 = amplitude (m)

Worked example: Calculating the total energy of oscillations

Calculating_Total_Energy_of_a_Simple_Harmonic_System_Worked_example_-_Calculating_Total_Energy_of_a_Simple_Harmonic_Syste, downloadable AS & A Level Physics revision notes

Step 1:            Write down all known quantities

Mass, m = 23 g = 23 × 10–3 kg

Amplitude, x0 = 1.5 cm = 0.015 m

Frequency, f = 4.8 Hz

Step 2:            Write down the equation for the total energy of SHM oscillations:

Calculating Total Energy of a Simple Harmonic System equation 1

Step 3:            Write an expression for the angular frequency

⍵ = 2πf = 2π× (4.8)

Step 4:            Substitute values into energy equation

Calculating Total Energy of a Simple Harmonic System Worked Example equation 2

E = 2.354 × 10–3 = 2.4 mJ (2 s.f)

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Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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