Conservation of Momentum
In a closed system the total momentum before a collision equals the total momentum after it.
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Play with the controls to see the idea in action.
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In a closed system the total momentum before a collision equals the total momentum after it.
- Applies to collisions and explosions
- Total momentum includes direction
- Used to find unknown speeds
🧠 Quick quiz
Three questions. Score 2 or more to complete the topic.
What is Conservation of Momentum?
Within Physics, which covers forces, energy, waves, electricity, magnetism, light and space explained through motion and experiments, this topic explains Conservation of Momentum. Physics topics explain how things move, push, heat up and carry energy, so each one here pairs a definition with a live model you can change.
Check that you can explain Conservation of Momentum in your own words and give one example from everyday life. If you can do that, you have understood the core idea.
Understanding Conservation of Momentum explains how the physical world behaves, from sport and transport to electricity and medicine.
Conservation of Momentum in detail
In a closed system with no external forces, the total momentum before an interaction equals the total momentum after. For two objects colliding, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂, with direction shown by positive and negative signs.
In explosions, objects start with zero momentum and fly apart in opposite directions, so a gun recoils backwards as the bullet goes forward. Momentum is conserved in all collisions, though kinetic energy is conserved only in perfectly elastic ones. This principle is used to analyse crashes, snooker shots and rocket propulsion.
How the interactive helps you understand Conservation of Momentum
Watch the force arrows in the interactive. Use any sliders or buttons it offers to change the values and see how the force arrows responds.
Before you change anything, predict the result. Then change one value at a time and check whether your prediction was right. This is a quick way to build a correct mental model of Conservation of Momentum.
Real-life uses and examples
- Use Conservation of Momentum to make sense of a practical and its results.
- Answer “explain” questions by linking Conservation of Momentum to a clear cause and effect.
- Predict what a ball, car or circuit will do with Conservation of Momentum before you do the calculation.
Common mistakes to avoid
- Mixing up Conservation of Momentum with a related quantity that has different units.
- Forgetting that some quantities, such as force or velocity, also have a direction.
- Learning the definition without testing it. Use the quiz and the interactive to check that you can apply the idea.
Frequently asked questions about Conservation of Momentum
What is Conservation of Momentum?
In a closed system the total momentum before a collision equals the total momentum after it.
What are the most important points about Conservation of Momentum?
The main points are: Applies to collisions and explosions; Total momentum includes direction; Used to find unknown speeds.
Why is Conservation of Momentum useful?
Understanding Conservation of Momentum explains how the physical world behaves, from sport and transport to electricity and medicine.
How can I practise Conservation of Momentum?
Open the interactive at the top of this page and change the controls one at a time. Predict the result before each change, then take the three-question quiz. A score of two or more marks the topic as complete on this device.
Which topics are related to Conservation of Momentum?
Related topics in Physics include Gravitation, Temperature and Heat, Thermal Conduction.