This topic summary covers Knowledge Organiser: Momentum within Momentum for GCSE Physics. Revise Momentum in Forces for GCSE Physics with 13 exam-style questions and 6 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Momentum
Key Terms
- Momentum: p = mv, vector, kg m/s
- Conservation: Total momentum unchanged in closed system
- Elastic collision: KE conserved too
- Inelastic collision: KE not conserved (momentum is)
Key Facts
- Momentum is a vector — direction matters
- Momentum ALWAYS conserved in closed system
- KE only conserved in elastic collisions
- Explosion from rest → equal and opposite momenta
Key Equations
- p = mv
- Total p before = Total p after
- Units: kg m/s (not N)
- Higher: F = Δ(mv)/Δt
Exam Tips
- Assign positive/negative directions first
- Write "momentum before = momentum after"
- State conservation explicitly in all explanations
- Momentum ≠ force — different units and concepts
Common Mistakes
- Not assigning direction signs: Momentum is a vector — always define a positive direction and use negative values for objects moving the opposite way before applying conservation
- Confusing momentum with kinetic energy: Momentum = mv (kg·m/s); KE = ½mv² (J) — they are different quantities with different units; momentum is always conserved in collisions, KE is not
- Forgetting to state conservation of momentum explicitly: In explanation questions, write "total momentum before = total momentum after" — don't just calculate without stating the principle
- Using wrong units: Momentum is in kg·m/s (or N·s) — not Newtons alone; force is in Newtons
- Getting the impulse equation wrong: F = Δp ÷ Δt means force equals change in momentum divided by time — a longer collision time means a smaller force for the same change in momentum
Practice questions for Momentum
What is the equation for momentum?
Explain what is meant by saying momentum is a vector quantity.
Quick recall flashcards
Key Facts About Momentum
Momentum is a VECTOR — has direction
Key Facts About Momentum
Unit: kg m/s (kilogram metres per second)