Conservation of Mechanical Energy (DP IB Physics)

Mechanical Energy

• Mechanical energy is the sum of kinetic energy, gravitational potential energy and elastic potential energy

Mechanical energy = 

• An example of a system that has mechanical energy is a spring and mass system

• The change in the total mechanical energy of a system should be interpreted in terms of the work done on the system by any non-conservative force

  • A non-conservative force is one that dissipates energy away from the system, such as friction

• When a vertical spring is extended and contracted, its energy is converted into other forms

• Although the total energy of the spring will remain constant, it will have changing amounts of:

  • Elastic potential energy (E_H or EPE)

  • Kinetic energy (E_k or KPE)

  • Gravitational potential energy (E_p or GPE)

• When a vertical mass is hanging on a spring and it moves up and down, its energy will convert between the three in various amounts

• For a horizontal mass on a spring system, there is no gravitational potential energy to consider because this is constant

  • The spring would only convert between kinetic and elastic potential energy

Conservation of Mechanical Energy

• In the absence of frictional, resistive forces, the total mechanical energy of a system is conserved

  • This means the total kinetic, gravitational potential and elastic potential energy is the same throughout the motion of the system

  • Because the total energy of a system is always conserved

• There are many scenarios that involve the transfer of kinetic energy into gravitational potential, or vice versa

• Some examples are:

  • A swinging pendulum

  • Objects in freefall

  • Sports that involve falling, such as skiing and skydiving

• Using the principle of conservation of energy, and taking any drag forces as negligible:

Loss in gravitational potential energy = Gain in kinetic energy

• Another example is if a ball on a spring oscillates vertically

• In this case:

Loss in gravitational potential energy = Gain in elastic potential energy

• The change in energy is the work done on the system. The types of changes depend on the system