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

The idea

Equilibrium questions ask you to keep two ideas apart. A book resting on a table is in equilibrium because the forces acting on the book, its weight downwards and the normal contact force from the table upwards, are equal and opposite, so the resultant force on the book is zero. Those two forces are not a third law pair: they act on the same object. The third law pairs are different. The book pushes down on the table and the table pushes up on the book; the Earth pulls down on the book and the book pulls up on the Earth. Each pair acts on two different objects, so a pair can never be responsible for equilibrium on its own. In a tug of war that nobody is winning, the resultant force on the rope is zero, while the pull of each team on the rope is paired with the rope's pull on that team.

What earns the marks

• Identifying the two forces acting on the object itself and stating they are equal and opposite.
• Stating that the resultant force on the object is therefore zero.
• Identifying the correct third law partner of a named force, acting on the other object.
• Stating that the two forces of a third law pair never act on the same object.

Common mistakes and how to put them right

• Calling the weight of a book and the table's contact force a Newton's Third Law pair.

• Assuming that because forces are equal and opposite they must be an interaction pair.

• Leaving out the object a force acts on, so the examiner cannot tell which pair is meant.

• Saying an object in equilibrium has no forces acting on it.

Exam tips

• Draw the object alone and mark only the forces acting on it; a third law partner never appears on that diagram.
• Use the phrase 'resultant force is zero' for equilibrium and 'equal and opposite on different objects' for the third law.
• Check the type of each force: the partner of a gravitational pull is another gravitational pull.