Knowledge Organiser: Impulse and Collisions
This topic summary covers Knowledge Organiser: Impulse and Collisions within Impulse & Collisions for GCSE Physics. Revise Impulse & Collisions in Forces for GCSE Physics with 13 exam-style questions and 10 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 6 of 6 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 6 of 6
Practice
13 questions
Recall
10 flashcards
Knowledge Organiser: Impulse and Collisions
Key Terms
- Impulse: The change in momentum of an object; equal to force × time (F × t)
- Momentum (p): Mass × velocity (kg m/s); a vector quantity
- Change in momentum (Δp): Final momentum minus initial momentum (kg m/s)
- Elastic collision: Collision where both momentum AND kinetic energy are conserved
- Inelastic collision: Collision where momentum is conserved but kinetic energy is not
Key Equations
- Impulse = F × t (units: N s)
- Impulse = Δp = mΔv (units: kg m/s)
- 1 N s = 1 kg m/s (equivalent units)
- F = Δp / t — longer time → smaller force for same momentum change
- Area under a force-time graph = impulse
Must-Know Facts
- Momentum is ALWAYS conserved in all collisions (closed system)
- Kinetic energy is ONLY conserved in elastic collisions
- In inelastic collisions, KE is converted to heat and sound
- Safety features (crumple zones, airbags, helmets) work by increasing collision time
- Increasing time → decreasing force for the SAME momentum change
Exam Tips
- Always show the momentum change calculation before finding force
- Include direction (sign) in momentum change calculations
- On force-time graphs: area under curve = impulse, NOT force or velocity
- Both N s and kg m/s are correct units for impulse — they are equal
- Elastic = bouncing apart (KE conserved); inelastic = sticking/deforming (KE lost)
Common Mistakes
- Forgetting direction in impulse calculations: Impulse and momentum are vectors — if an object bounces back, the change in momentum is 2mv (not zero), because direction reverses
- Reading force-time graphs incorrectly: The area under a force-time graph gives impulse (N·s), not force or velocity — do not read off the peak force as the impulse
- Confusing elastic and inelastic collisions: In both types, total momentum is conserved — only kinetic energy is conserved in elastic collisions; in inelastic collisions some KE converts to heat/sound
- Not calculating Δp before force: Always find the change in momentum first (Δp = mv − mu), then divide by time to find the force — don't try to find force directly
- Thinking longer collision time means more impulse: Longer collision time means smaller force for the same impulse — the impulse (change in momentum) stays the same regardless of collision duration
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Practice Questions for Impulse & Collisions
Which equation correctly defines impulse?
Explain how an airbag reduces the risk of injury to a driver in a collision.
Quick Recall Flashcards
13 questions on Impulse & Collisions — practise free
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