Newton’s Third Law of Motion

Key Statement: “To every action, there is always an equal and opposite reaction.” Forces always occur in pairs between two interacting bodies.

Important Points

  • 💡 Action and reaction forces are equal in magnitude, opposite in direction, and act on different bodies. For example:
    • If body A exerts a force FAB on body B, then body B exerts a force -FBA on body A: \[ \mathbf{F}_{AB} = -\mathbf{F}_{BA} \]
  • ⏳ Forces act simultaneously. There’s no delay between action and reaction.
  • 🚫 Do not add action and reaction forces for the same body. They act on different objects!

Examples to Understand

  • Batsman and Ball:
    • Impulse (change in momentum) = \( 0.15 \times 12 – (-0.15 \times 12) = 3.6 \, \text{N s} \).
    • Direction: From batsman to bowler.
  • Billiard Balls Hitting a Wall:
    • Force on the wall is always normal (perpendicular) to the wall, regardless of the ball’s angle.
    • Impulse ratio for two angles (Case a vs. Case b): \[ \frac{2mu}{2mu \cos 30^\circ} = \frac{2}{\sqrt{3}} \approx 1.2 \]

Conservation of Momentum

Why it happens: Newton’s third law ensures internal forces in a system cancel out, leaving total momentum unchanged.

  • Example: Firing a bullet from a gun:
    • Bullet’s forward momentum = Gun’s backward recoil momentum.
    • Total momentum before and after firing: \[ \mathbf{p}_{\text{bullet}} + \mathbf{p}_{\text{gun}} = 0 \]

Important NEET Concepts

  • 🔑 Force Pairs: Every force has an equal and opposite counterpart (e.g., Earth and stone gravity interaction).
  • 🔑 Impulse Calculations: Use momentum change (\( \Delta p = m \Delta v \)) for problems like balls hitting walls.
  • 🔑 Conservation of Momentum: Critical for collision/recoil problems (e.g., bullet-gun systems).
  • 🔑 Direction of Forces: In collisions, forces are always normal to the surface (billiard ball example).