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, not newtons. Force is in newtons.
- Getting the force and momentum 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
Memory Aid
- p = mv — "Pretend Mass is Velocity's bodyguard": Momentum (p) is calculated when mass (m) and velocity (v) work together. The unit kg m/s reminds you it includes both kg (mass) and m/s (velocity).
- Conservation — "The Ledger Rule": Think of momentum like money in a ledger. It can be transferred between objects (like transferring money between accounts) but the TOTAL amount never changes — it's always balanced.
- Direction tip: Pick one direction as positive (e.g., right = positive, left = negative). An object moving left has negative momentum. Stick to your chosen convention throughout the whole problem!