Newton's Third Law
Newton's third law states that when one object exerts a force on another, the second exerts an equal and opposite force back.
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Newton's third law states that when one object exerts a force on another, the second exerts an equal and opposite force back.
- Action and reaction act on different objects
- Rockets push gas down and are pushed up
- The forces are equal in size
🧠 Quick quiz
Three questions. Score 2 or more to complete the topic.
What is Newton's Third Law?
Within Physics, which covers forces, energy, waves, electricity, magnetism, light and space explained through motion and experiments, this topic explains Newton's Third Law. 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 Newton's Third Law in your own words and give one example from everyday life. If you can do that, you have understood the core idea.
Understanding Newton's Third Law explains how the physical world behaves, from sport and transport to electricity and medicine.
Newton's Third Law in detail
Newton's third law states that when object A exerts a force on object B, B exerts an equal and opposite force on A. The two forces are the same type, act on different objects and have the same size at the same time.
A rocket pushes gas backward and the gas pushes the rocket forward; a swimmer pushes water back and is pushed forward. A common confusion is thinking the forces cancel, but they act on different objects so they do not. Distinguish these from balanced forces on one object.
How the interactive helps you understand Newton's Third Law
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 Newton's Third Law.
Real-life uses and examples
- Predict what a ball, car or circuit will do with Newton's Third Law before you do the calculation.
- Explain everyday devices, such as brakes, lifts or chargers, using Newton's Third Law.
- Use Newton's Third Law to make sense of a practical and its results.
Common mistakes to avoid
- Mixing up Newton's Third Law 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 Newton's Third Law
What is Newton's Third Law?
Newton's third law states that when one object exerts a force on another, the second exerts an equal and opposite force back.
What are the most important points about Newton's Third Law?
The main points are: Action and reaction act on different objects; Rockets push gas down and are pushed up; The forces are equal in size.
Why is Newton's Third Law useful?
Understanding Newton's Third Law explains how the physical world behaves, from sport and transport to electricity and medicine.
How can I practise Newton's Third Law?
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 Newton's Third Law?
Related topics in Physics include Air Resistance and Terminal Velocity, Centre of Mass and Stability, Pressure in Liquids.