Students should be able to apply Newton’s Third Law to examples of…

The idea

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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Treating the weight of a book and the normal contact force from a table as a Third Law pair.

  Put it right 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.

  Put it right by explaining that forces on different objects cannot cancel each other.

• Assuming equilibrium requires no forces at all.

  Put it right by stating that equilibrium occurs when the resultant force is zero, even though individual forces act.

Exam tips

• 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.