Newton's Third Law — AQA GCSE Combined Science
Test yourself on Newton's Third Law with AQA GCSE practice questions.
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Newton's Third Law explained
Newton’s Third Law describes what happens whenever two objects interact: the two objects push or pull on each other, and the two forces are equal in size and opposite in direction.
Read the full explanation
The two forces act on different objects, so they never cancel out on a single free-body diagram. A useful method is to name the pair clearly: “force of A on B” and “force of B on A”. For example, a swimmer pushes water backwards with her hands; the water pushes the swimmer forwards with an equal-sized force. Similarly, a book resting on a table pushes down on the table, and the table pushes up on the book. The law applies to contact forces such as friction and normal contact force, and to non-contact forces such as gravity and magnetic force.
Whenever two objects interact, the forces they exert on each other are equal and opposite.
This statement is the full form of Newton’s Third Law. Whenever two objects interact, object A exerts a force on object B, and object B exerts a force on object A. The two forces are equal in magnitude and opposite in direction. They are always the same type of force: both gravitational, both contact, or both magnetic, for example. The forces act on different objects, so they do not cancel each other out. A reliable method is to write the pair as “force of A on B” and “force of B on A”, then check that the sizes are equal and the arrows point in opposite directions. For example, when a rocket pushes exhaust gases downwards, the gases push the rocket upwards with an equal force.
Students should be able to apply Newton’s Third Law to examples of equilibrium situations.
Newton’s Third Law states that when two objects interact, they exert equal and opposite forces on each other. These two forces act on different objects, so they do not cancel. In an equilibrium situation, such as a book resting on a table, the forces on the book balance: its weight acts downwards and the table pushes upwards with an equal force. The book pushes down on the table, and the table pushes up on the book; these are the Third Law pair. To apply the law, identify the two interacting objects, name the force each exerts on the other, and check they are equal in size and opposite in direction. Equilibrium means the resultant force on each object is zero, so it stays at rest or moves at constant velocity.
Your focus
- State that when two objects interact, the forces they exert on each other are equal in size and opposite in direction.
- Identify the two objects in an interaction and name the force each exerts on the other.
- Explain why a pair of Newton’s Third Law forces does not cancel out when considering the motion of one object.
Show all 9 objectives
- State Newton’s Third Law for two interacting objects.
- Describe a named interaction using a correctly identified pair of equal and opposite forces.
- Distinguish between a Newton’s Third Law force pair and a pair of balanced forces acting on one object.
- Identify the two interacting objects and the paired forces in a simple equilibrium situation.
- Explain why equal and opposite Third Law forces do not cancel because they act on different objects.
- Use the idea of zero resultant force to describe why an object in equilibrium stays at rest or moves at constant velocity.
Newton's Third Law exam tips
Marking Points
- State that an interaction involves two objects, each exerting a force on the other.
- State that the two forces are equal in magnitude.
- State that the two forces act in opposite directions.
- Identify the forces as a pair acting on different objects, for example force of A on B and force of B on A.
- Apply the law to a named example, such as a swimmer pushing water backwards while the water pushes the swimmer forwards.
- Recognise that the two forces do not cancel because they act on different objects.
- State that two interacting objects exert forces on each other.
- State that the two forces are equal in magnitude.
- State that the two forces are opposite in direction.
- Identify the forces as acting on different objects, for example force of A on B and force of B on A.
- Recognise that the two forces are the same type, such as both gravitational or both contact forces.
- Apply the law to a named interaction, such as a rocket pushing gases downwards while the gases push the rocket upwards.
- State that the two forces in a Newton’s Third Law pair are equal in magnitude and opposite in direction.
- Explain that the paired forces act on two different objects, so they do not cancel each other.
- Identify the interacting objects in a given equilibrium example, such as a book and a table or a swimmer and water.
- Describe the force each object exerts on the other, for example the table pushes up on the book and the book pushes down on the table.
- Link equilibrium to a zero resultant force on each object, meaning it remains at rest or moves at constant velocity.
- Distinguish a Third Law pair from a balanced pair of forces acting on the same object.
Examiner Tips
- 💡Name both objects in the interaction before stating the two forces, for example “the Earth pulls the Moon” and “the Moon pulls the Earth”.
- 💡Use the wording “equal in magnitude and opposite in direction” rather than “the same force” to show the forces are separate.
- 💡When explaining motion, check whether the forces act on the same object; if they act on different objects, do not treat them as a balanced pair.
- 💡Write the force pair in the form “force of X on Y” and “force of Y on X” to make the two objects clear.
- 💡Check that both forces are the same type, for example both gravitational, before describing them as a Newton’s Third Law pair.
- 💡Use a labelled diagram with arrows of equal length pointing in opposite directions to support your explanation.
- 💡Name the two objects involved before describing the paired forces, so the examiner can see the interaction clearly.
- 💡Use the wording ‘object A exerts a force on object B, and object B exerts an equal and opposite force on object A’ to make the pairing explicit.
- 💡When a question says ‘equilibrium’, state that the resultant force is zero and link this to constant velocity or rest.
Common Mistakes
- Saying the forces cancel out: correct this by stressing that the two forces act on different objects, so they cannot cancel on one object.
- Describing a pair of forces on the same object, such as weight and normal contact force on a book: correct this by identifying the two interacting objects and naming each force with its object pair.
- Assuming the forces are equal only when the objects are stationary: correct this by applying the law to moving objects, such as a car tyre pushing the road backwards while the road pushes the car forwards.
- Treating the two forces as acting on the same object and therefore balanced: correct this by identifying the two different objects and the force each exerts on the other.
- Mixing up Newton’s Third Law pairs with balanced forces on one object: correct this by checking whether the two forces act on the same object or on different objects.
- Stating that the forces are equal and opposite but forgetting to say they act on different objects: correct this by always naming both objects in the pair.
- Treating the weight of a book and the normal contact force from a table as a Third Law pair; correct this by noting both act on the book, whereas a Third Law pair acts on two different objects.
- Saying the equal and opposite forces cancel out; correct this by explaining that forces on different objects cannot cancel each other.
- Assuming equilibrium requires no forces at all; correct this by stating that equilibrium occurs when the resultant force is zero, even though individual forces act.