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School-college Physics Notes: Forces & motion Section 6.3 Momentum effects - calculations

GCSE level Physics exam revision notes on Forces & Motion Part 6

Forces & Newton's Laws of Motion

6.3 Other examples of momentum effects - bullet & rocket

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics [forces-motion-6- updated Mar 28th 2026]

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INDEX physics notes: Conservation of momentum, elastic & non-elastic collisions, Newton's 2nd law calculations

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6.3 Other examples of momentum effects

The above examples involve collision but there are all sorts of momentum situations e.g.

When a gun is fired the bullet goes in the forward direction (positive momentum), but the gun recoils in the complete opposite direction (negative momentum).

From Newton's 3rd law, you get an equal and opposite force reaction.

The bullet goes one way and the gun the other way from the force of the explosion.

numerically, ignoring +/- signs: mgunvgun = mbulletvbullet     (see Q2 in part 6.4)

Note that the combined momentum = zero, which it is at the start and immediately after the gun is fired.

 

Before a rocket is fired it has zero velocity and zero momentum.

When the rocket's propellant is fired, the rocket goes one way (positive momentum) and the exhaust gases go in the opposite direction (negative momentum).

So, in conserving momentum, we can say ..

numerically, ignoring +/- signs: mrocketvrocket = mexhaust gasesvexhaust gases 

Because the rocket produces gases with great force and velocity, the rocket must respond according to Newton's 3rd law of motion - equal and opposite forces in dramatic action!

 

INDEX physics notes: Conservation of momentum, elastic and non-elastic collisions, Newton's 2nd law calculations - problem solving

Key points force and motion: Examples of the conservation of momentum

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured set of GCSE/IGCSE Physics revision notes on examples of the Law of Conservation of Momentum, tailored to the major UK exam boards: AQA, Edexcel, OCR, WJEC, CCEA, and CIE.

These notes and examples focus on qualitative understanding, concepts, not calculations.

Law of Conservation of Momentum: Recap

  • In a closed system (no external forces), the total momentum before an event equals the total momentum after.
  • Applies to collisions, explosions, and recoil events.
  • Momentum is a vector quantity — direction matters.

Real-World Examples (Non-Calculative)

1. Ice Skaters Colliding

  • One skater pushes another on ice.
  • After collision, both move together.
  • Momentum is transferred and conserved.

2. Gun Recoil

  • A bullet is fired forward; the gun recoils backward.
  • Momentum of bullet and gun are equal and opposite.
  • Demonstrates Newton’s Third Law and conservation.

3. Newton’s Cradle

  • Steel balls transfer momentum through a series of collisions.
  • End ball swings out with same momentum.
  • Ideal example of elastic collisions and conservation.

4. Rocket Propulsion

  • Gases expelled backward; rocket moves forward.
  • Momentum of gases and rocket are equal and opposite.
  • Explains propulsion in space.

5. Car Crash (Closed System Assumption)

  • Two vehicles collide and move off together or separately.
  • Momentum before and after is conserved (ignoring friction).

6. Exploding Firework

  • Firework explodes into fragments.
  • Total momentum of fragments equals original firework’s momentum.

Common Misconceptions

  • “Momentum is always conserved” → Only in closed systems.
  • “Momentum is scalar” → False: it’s a vector.
  • “If objects stop moving, momentum is lost” → Momentum is transferred, not lost.
  • “Only collisions involve momentum” → Explosions and recoil events count too.

Student Exam Tips

  • Focus on direction and system boundaries.
  • Use diagrams to visualise before and after scenarios.
  • In written answers, state the principle clearly.
  • Avoid assuming energy is conserved — momentum and energy are separate.
  • Link examples to Newton’s laws for deeper understanding.

Keywords, phrases and learning objectives for momentum

Be able to analyse in terms of momentum effects, what happens when a rocket or is fired.

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INDEX physics notes: Conservation of momentum, elastic and non-elastic collisions, Newton's 2nd law calculations - problem solving

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