Key Terms
- Impulse: The change in momentum of an object; equal to force ร time (F ร t)
- Momentum (p): Mass ร velocity (kg m/s); a vector quantity
- Change in momentum (ฮp): 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 ร t (units: N s)
- Impulse = ฮp = mฮv (units: kg m/s)
- 1 N s = 1 kg m/s (equivalent units)
- F = ฮp / t โ longer time โ 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 โ 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 โ 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 โ 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 (Nยทs), not force or velocity โ do not read off the peak force as the impulse
- Confusing elastic and inelastic collisions: In both types, total momentum is conserved โ only kinetic energy is conserved in elastic collisions; in inelastic collisions some KE converts to heat/sound
- Not calculating ฮp before force: Always find the change in momentum first (ฮp = mv โ mu), then divide by time to find the force โ don't try to find force directly
- Thinking longer collision time means more impulse: Longer collision time means smaller force for the same impulse โ the impulse (change in momentum) stays the same regardless of collision duration
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. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 7 of 7 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Understanding Impulse
Impulse = Force ร time (F ร t) โ measured in N s
Understanding Impulse
Impulse = Change in momentum (ฮp = mฮv) โ measured in kg m/s