The Principle of Conservation of Energy (Edexcel A Level Physics)

Revision Note

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Lindsay Gilmour



The Principle of Conservation of Energy

  • The principle of conservation of energy is a law of Physics which always applies to a closed system


  • To apply conservation of energy, heat losses are usually ignored during the calculation stage
  • In reality there are always some energy losses from the system
    • These should be mentioned when comparing calculated, ideal values to real-life situations

  • Conservation of energy is often applied in questions about exchanges between kinetic energy and gravitational energy
  • Common examples include:
    • A swinging pendulum
    • Objects in free fall 
    • Sports such as skiing or skydiving where gravity is causing motion and few drag forces apply

  • The gravitational potential energy stored initially is transferred to kinetic energy, or vice versa
  • This allows either;
    • Final velocity to be found from the distance the object moved, or
    • Height of a drop from the final velocity

Worked example

The diagram below shows a skier on a slope descending 750 m at an angle of 25° to the horizontal.Kinetic GPE Transfer Worked Example, downloadable AS & A Level Physics revision notesCalculate the final speed of the skier, assuming that he starts from rest and 15% of his initial gravitational potential energy is not transferred to kinetic energy.

Step 1: Write down the known quantities

Kinetic GPE Transfer Worked Example Ans, downloadable AS & A Level Physics revision notes

    • Vertical height, h = 750 sin 25°
    • Ek = 0.85 Ep

Step 2: Equate the equations for Ek and Egrav

Ek = 0.85 Egrav

½ mv2 = 0.85 × mgh

Step 3: Rearrange for final speed, v

Step 4: Calculate the final speed, v

Exam Tip

Gravitational energy:

  • This equation only works for objects close to the Earth’s surface where we can consider the gravitational field to be uniform.

Kinetic energy:

  • When using the kinetic energy equation, note that only the speed is squared, not the mass or the ½.
  • If a question asks about the ‘loss of kinetic energy’, remember not to include a negative sign since energy is a scalar quantity.

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.