Newton's third law

The law

If body A exerts a force on body B, then B exerts a force on A that is equal in size and opposite in direction.

Properties of an interaction pair

The two forces:

• are equal in magnitude
• act in opposite directions along the same line
• are the same type of force (both gravitational, both contact, and so on)
• act on different bodies

Not the same as balanced forces

Two balanced forces in equilibrium act on the same body. A book on a table has weight (Earth pulls book down) and a normal force (table pushes book up). These balance, but they are not a third-law pair. The pair to the weight is the book pulling the Earth up. The pair to the normal force is the book pushing down on the table.

Worked example

Two ice skaters push apart. A (50 kg) pushes B (75 kg) with 150 N. Find the force on A and the acceleration of each.

1. By the third law, B pushes A with 150 N in the opposite direction.
2. a_A = 150 / 50 = 3.0 m/s²
3. a_B = 150 / 75 = 2.0 m/s²

Answer: Both forces are 150 N; A accelerates at 3.0 m/s² and B at 2.0 m/s²

Check yourself

1. The Earth pulls a ball downwards. What is the Newton's third law pair to this force?
   • The ground pushes the ball upwards
   • Air resistance on the ball
   • The ball pushes the ground downwards
   • The ball pulls the Earth upwards
   Show answer

   The ball pulls the Earth upwards. The pair must be the same type (gravitational) and act on the other body (the Earth).

2. Which statement about a Newton's third law pair is FALSE?
   • The forces are equal in size
   • The two forces act on the same body
   • The forces are of the same type
   • The forces act in opposite directions
   Show answer

   The two forces act on the same body. Third-law pairs always act on different bodies.

3. Why are the weight of a book and the normal force from the table NOT a third-law pair?
   • They are not equal in size
   • They act in the same direction
   • The normal force is not a real force
   • They act on the same body and are different types of force
   Show answer

   They act on the same body and are different types of force. Both act on the book, and one is gravitational while the other is a contact force.

4. A 60 kg skater pushes against a wall with a force of 120 N. What force, in N, does the wall exert on the skater?
   Show answer

   120 N. By Newton's third law the wall pushes back with an equal force, 120 N.

5. A swimmer pushes water backwards with her hands. What makes her move forwards?
   • Her weight
   • The upthrust from the water
   • The water pushes her forwards with an equal force
   • Her momentum
   Show answer

   The water pushes her forwards with an equal force. The water exerts an equal and opposite force on her.

6. Two trolleys push apart with forces of 12 N. Trolley X has mass 0.80 kg. Find the size of its acceleration in m/s².
   Show answer

   15 m/s². a = F / m = 12 / 0.80 = 15 m/s².

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.