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First exams 2025

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Kinetic Energy (SL IB Physics)

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Ashika

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Ashika

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Kinetic Energy

  • Kinetic energy (Ek) is the energy an object has due to its translational motion (i.e. because it's moving)
    • The faster an object is moving, the greater its kinetic energy

  • When an object is falling, it is gaining kinetic energy since it is accelerating under gravity
  • This energy is transferred from the gravitational potential energy it is losing
  • An object will maintain this kinetic energy unless its speed or mass changes

  • Kinetic energy can be calculated using the following equation:

E subscript k space end subscript space equals space 1 half m v squared

  • Where:
    • Ek = kinetic energy (J)
    • m = mass (kg)
    • v = velocity (m s–1)

Kinetic energy diagram, downloadable AS & A Level Physics revision notes

Kinetic energy: The energy an object has when it is moving

  • Another quantity that also depends on mass m and velocity v is momentum
  • Therefore, kinetic energy can be written in terms of momentum p, using the equation

E subscript k space equals space fraction numerator p squared space over denominator 2 m end fraction

  • Where:
    • p = momentum (kg m s–1)

 

  • This form is very useful in particle physics, when comparing the momentum and kinetic energy of a particle

Worked example

A body travelling with a speed of 12 m s–1 has kinetic energy 1650 J. The speed of the body is increased to 45 m s–1.

Estimate the body's new kinetic energy.

WE - kinetic energy answer image, downloadable AS & A Level Physics revision notes

Examiner Tip

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.

Both variations of the kinetic energy equation are given in your data booklet. You will most likely use 1 half m v squared in a mechanics question, and fraction numerator p squared over denominator 2 m end fraction in particle physics.

If you are not convinced these are in fact the same equation:

E subscript k space equals fraction numerator space p squared over denominator 2 m end fraction space equals space fraction numerator open parentheses m v close parentheses squared space over denominator 2 m end fraction space equals space fraction numerator m squared v squared space over denominator 2 m end fraction space equals fraction numerator m v squared space over denominator 2 end fraction space equals space 1 half m v squared

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    Ashika

    Author: Ashika

    Expertise: Physics Project Lead

    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.