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Dynamics — Lesson 2 of 9  |  Syllabus ref: O Level Physics (G3, 6091) — Section 4: Dynamics  |  Prerequisite: D1 — What is a Force and Free Body Diagrams

By the end of this lesson, you should be able to…

  • State Newton’s Third Law and identify action-reaction pairs
  • Use the ABBA pattern to find the paired force for any given force
  • Apply the three criteria that all action-reaction pairs must satisfy
  • Distinguish between action-reaction pairs and balanced forces acting on the same object

D2 — Newton’s 3rd Law: Action and Reaction Pairs (4:40)

1. Newton’s Third Law

Newton’s Third Law When body A exerts a force on body B, body B exerts an equal and opposite force on body A.

Every force has a “partner” force. These two paired forces are called an action-reaction pair. The key thing is that the two forces in the pair act on different objects — one on A, one on B.

Example: Water bottle on a table

water bottle on table label two arrows: (1) bottle pushes down on table, (2) table pushes up on bottle

The bottle pushes down on the table (normal contact force).
By Newton’s 3rd Law, the table pushes up on the bottle (normal contact force).
These two forces are an action-reaction pair.

2. How to Identify Action-Reaction Pairs: The ABBA Pattern

To find the action-reaction partner of any given force, use the ABBA pattern. Write the force as a sentence in the form “force exerted on [Object A] by [Object B]”, then swap the two objects.

The ABBA Pattern
Force exerted on bottle by table
↓
Action-reaction pair: force exerted on table by bottle

The two objects simply swap places in the sentence. The type of force stays the same.

⚠ Common mistake Students often identify an action-reaction pair by spotting two forces that are “equal and opposite.” But equal and opposite is the conclusion, not the way to identify the pair. Use the ABBA pattern to identify — then state that the forces are equal in magnitude and opposite in direction as a consequence of Newton’s 3rd Law.

3. Three Criteria for Action-Reaction Pairs

An action-reaction pair of forces must satisfy all three of the following:

Checklist

  1. Equal in magnitude, opposite in direction. The forces are the same size and point in opposite directions.
  2. Acting on different objects. One force acts on A, the other on B. They will never appear on the same free body diagram.
  3. Same type of force. If one is a normal contact force, the other must also be a normal contact force. Friction pairs with friction; gravitational pairs with gravitational.

If you use the ABBA pattern correctly, criteria 1 and 2 are automatically satisfied. Always double-check criterion 3 explicitly.

4. Balanced Forces vs. Action-Reaction Pairs

This is one of the most common points of confusion in this topic:

⚠ Not the same thing Balanced forces act on the same object and cancel out (net force = 0).
Action-reaction pairs act on different objects and can never cancel each other.

Worked Examples

Worked Example 1
A person pushes horizontally against a wall. State the action-reaction pair force to the force the person exerts on the wall.

Given force: Normal contact force exerted on the wall by the person’s hand (directed into the wall).

ABBA swap: Normal contact force exerted on the person’s hand by the wall (directed away from the wall, back towards the person).

The wall exerts a real force on the person. This is why you feel resistance when you push a wall — the wall is literally pushing back on your hand.

person pushing wall arrow from hand into wall, arrow from wall back onto hand
Worked Example 2
A rocket accelerates upward by expelling gases downward. Identify the action-reaction pair that causes the rocket to move.

Force 1: The rocket exerts a force on the gases (pushing them downward out of the engine).

ABBA swap: The gases exert a force on the rocket (pushing it upward).

It is this upward force from the gases on the rocket that propels the rocket forward. Note: the rocket and the gases are the two objects. There is no “wall” or external surface needed — this is why rockets work in the vacuum of space.

rocket with exhaust arrow down (force on gas) and arrow up (force on rocket from gas)

Try It Yourself

Question 1

A horse pulls a cart forward. A student argues: “By Newton’s 3rd Law, the cart pulls the horse backward with an equal force. So the net force is zero and the horse-cart system can never accelerate.”

Identify the flaw in this argument.

The student has confused action-reaction pairs with balanced forces. The horse pulls the cart forward, and the cart pulls the horse backward — these are an action-reaction pair, acting on different objects. They cannot cancel each other because cancellation only happens when forces act on the same object.

To find the net force on the horse, we look at all forces on the horse alone (e.g. friction from the ground pushing the horse forward vs the cart pulling it backward). To find the net force on the cart, we look at all forces on the cart alone. Each object is analysed separately using its own free body diagram.

Question 2

A student stands on a skateboard and pushes against a wall. Both the student and the skateboard move away from the wall. Using Newton’s 3rd Law, explain why the student moves away from the wall.

When the student pushes against the wall, the student exerts a force on the wall (directed towards the wall). By Newton’s 3rd Law, the wall exerts an equal and opposite force on the student (directed away from the wall). This reaction force from the wall on the student is an unbalanced force, which causes the student (and skateboard) to accelerate away from the wall.

Further Resources