Key Terms
- Force (F): The rate of change of momentum; change in momentum á time (N)
- Momentum (p): Mass Ă velocity (kg m/s); a vector quantity
- Change in momentum (Îp): 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
- p = m Ă v (units: kg m/s)
- Îp = mv â mu (units: kg m/s)
- F = Îp á t: longer time â smaller force for the same change in momentum
- Îp = F Ă t: the change in momentum equals the force multiplied by the time it acts
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, seat belts, crash mats, helmets) work by increasing the collision time
- Increasing time â decreasing force for the SAME change in momentum
Exam Tips
- Always show the change in momentum calculation before finding force
- Include direction (sign) in momentum change calculations
- Momentum is measured in kg m/s, force in N and time in s
- Write the equation, substitute, then give the answer with its unit
- Elastic = bouncing apart (KE conserved); inelastic = sticking/deforming (KE lost)
Common Mistakes
- Forgetting direction in momentum calculations: Momentum is a vector â if an object bounces back at the same speed, the change in momentum is 2mv (not zero), because the direction reverses
- Mixing up force and momentum: Force is the rate of change of momentum (change in momentum á time). A big change in momentum does not always mean a big force, because the time matters too
- Confusing elastic and inelastic collisions: In both types, total momentum is conserved â only kinetic energy is conserved in elastic collisions; in inelastic collisions some KE converts to heat/sound
- Not calculating Îp before force: Always find the change in momentum first (Îp = mv â mu), then divide by time to find the force â don't try to find force directly
- Thinking a longer collision time means a bigger change in momentum: Longer collision time means a smaller force for the same change in momentum â the change in momentum stays the same regardless of how long the collision lasts
This topic summary covers Knowledge Organiser: Force and Collisions within Force & Collisions for GCSE Physics. Revise Force & Collisions in Forces for GCSE Physics with 10 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 Force & Collisions
Which equation links force, change in momentum and time?
Explain how an airbag reduces the risk of injury to a driver in a collision.
Quick recall flashcards
Force and Change in Momentum
Change in momentum = force Ă time (Îp = F Ă t)
Force and Change in Momentum
Change in momentum = final momentum minus initial momentum (Îp = mv â mu), measured in kg m/s