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1.4.4Dynamics

Newton’s Third Law — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 5 parts Lesson 14 of 22

📚 SQA National 5 Physics Hub / Topical Revision
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Curriculum Equation Key Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: Force = Mass × Acceleration
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Part 1 of 5The Law

1.4.4 Newton's Third Law

Forces never act alone; they come in pairs between two interacting objects.

Statement
  • If object A exerts a force on object B, then B exerts an equal and opposite force on A.
Pairs
  • The two forces of an action–reaction pair always have the same size, the opposite direction and the same type.
Different objects
  • The pair acts on two different objects, which is why they do not cancel each other out.
Both accelerate
  • Both objects feel a force, but their accelerations differ if their masses are different, since a = F ÷ m.

Part 2 of 5Examples

Examples
Rocket
  • The rocket pushes hot gas downwards, and the gas pushes the rocket upwards with an equal force called thrust.
Walking
  • Your foot pushes backwards on the ground and the ground pushes forwards on you, moving you ahead.
Swimming
  • The swimmer pushes water backwards and the water pushes the swimmer forwards.
Collision
  • When two skaters push apart each feels the same size of force, but the lighter skater accelerates more.
ActionReaction
Rocket pushes gas downGas pushes rocket up
Hand pushes wallWall pushes hand back
Earth pulls on youYou pull on the Earth

Part 3 of 5Avoiding Confusion

1
Write the pair in the form: A exerts a force on B, and B exerts a force on A.
2
Check that the two forces act on different objects; forces on the same object are not a third-law pair.
3
Remember a book on a table: its weight and the table's reaction are balanced, but they are not an action–reaction pair.
4
The true pair for the weight of the book is the book's gravitational pull on the Earth.

Part 4 of 5Worked Example

Question

  • Two ice skaters at rest push apart.
  • Skater A has mass 60 kg and skater B has mass 40 kg.
  • A pushes B with a force of 120 N for a short time.
  • Find the acceleration of each.

  1. By the third law, B pushes A with a force of 120 N in the opposite direction.
  2. a(A) = F ÷ m = 120 ÷ 60 = 2.0 m/s².
  3. a(B) = 120 ÷ 40 = 3.0 m/s², in the opposite direction.

Answer A accelerates at 2.0 m/s² and B at 3.0 m/s² in the opposite direction.

Part 5 of 5Exam Checklist & Tips

1.4.4 – Key Points for the Exam

  • Action–reaction pair Two equal and opposite forces between two interacting objects.
  • Thrust The forward force on a rocket or jet produced by expelling gas backwards.
  • Newton's third law Every action force has an equal and opposite reaction force.
  • Interaction The mutual forces exerted by two objects on each other.

Common Mistakes

  • Believing the two forces cancel; they act on different objects, so they cannot cancel.
  • Thinking the larger object feels the larger force; both forces are equal in size.
  • Treating weight and the reaction force of a table as a third-law pair.
  • Forgetting that equal forces on unequal masses give unequal accelerations.

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