Edexcel A Level (IAL) Physics-1.12 Newton’s Third Law of Motion- Study Notes- New Syllabus

Newton’s Third Law of Motion and Properties of Interaction Pairs

Newton’s Third Law of Motion

Newton’s third law states that:

“For every action, there is an equal and opposite reaction.”

This law describes the forces that two bodies exert on each other during an interaction.

Key Statement in Physics Form:

If body A exerts a force on body B, then body B exerts an equal and opposite force on body A.

Symbolically:

\( \vec{F}_{AB} = -\vec{F}_{BA} \)

• \( \vec{F}_{AB} \) = force exerted by A on B
• \( \vec{F}_{BA} \) = force exerted by B on A

Properties of Newton’s Third Law Force Pairs

• The forces are equal in magnitude.
• The forces are opposite in direction.
• The forces act on two different bodies (never on the same body).
• The forces are of the same type (e.g., both gravitational, both normal, both magnetic).
• Both forces occur simultaneously, not one after the other.

Examples of Action–Reaction Pairs

• A book on a table:
  
  • Book pushes the table down.
  • Table pushes the book up (equal and opposite).
• A rocket launching:
  
  • Rocket pushes gas downwards.
  • Gas pushes rocket upwards.
• Walking:
  
  • Foot pushes ground backwards.
  • Ground pushes foot forward.

Common Misconceptions

• Action–reaction forces do NOT cancel out because they act on different bodies.
• They are not cause–effect; they occur simultaneously.
• Normal reaction is NOT the third-law pair of weight.
  
  • Weight is Earth pulling object downward.
  • Reaction pair: object pulls Earth upward.