Energy in SHM (CIE A Level Physics)

Equations for Kinetic and Gravitational Potential Energy

• Recall that the kinetic energy is defined by the equation:

KE = mv²

• Where:
  • KE = kinetic energy (J)
  • m = mass of oscillating object (kg)
  • v = velocity of oscillating mass (ms⁻¹)

• Gravitational potential energy is defined by the equation:

ΔGPE = mgΔh 

• Where:
  • GPE = gravitational potential energy (J)
  • m = mass of oscillating object (kg)
  • g = gravitational field strength (Nkg⁻¹)
  • h = change in height of oscillating mass (m)

Energy Transfer in SHM

• 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 system has maximum kinetic energy when the displacement is zero because the oscillator is at its equilibrium position and moving at maximum velocity.

•  Therefore, the kinetic energy is zero at maximum displacement
• • When displacement equals the amplitude of oscillation. x = x₀:

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

• 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

Energy-Time Graph

Graph of Changing Kinetic and Potential Energies

The kinetic and potential energy of an oscillator in SHM vary periodically

• The key features of the energy-time graph are:
  • 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)

Energy-Displacement Graph

• The energy-displacement graph for half a cycle looks like:

The Energy-Displacement Graph for Half a Cycle

Potential and kinetic energy v displacement in half a period of an SHM oscillation

• 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

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

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