5.7.3Forces
Collisions — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 44 of 44
Part 1 of 4Force and change in momentum
1Momentum changes
- Momentum changes In a collision an object's momentum changes from m × u to m × v.
2Rate of change
- Rate of change Force equals change in momentum divided by time taken: F = (m × Δv) ÷ Δt.
3Longer time, smaller force
- Longer time, smaller force If the same change in momentum happens over a longer time, the average force is smaller.
4Less harm
- Less harm A smaller force on a person means less chance of serious injury.
Part 2 of 4Safety features
| Feature | How it works | Result |
|---|---|---|
| Crumple zone | Front and rear sections deform in a collision, increasing the time over which the car stops | Smaller force on the occupants |
| Seat belt | Stretches slightly and holds the person in place, so they slow with the car | Longer stopping time; prevents hitting the windscreen |
| Airbag | Inflates and cushions the head and chest, spreading the force over a larger area | Longer stopping time and lower pressure |
| Cycle helmet | The foam lining crushes on impact | Increases the time for the head to stop |
| Padded surfaces | Playgrounds and mats compress under a falling child | Slow the stop, reducing force |
Part 3 of 4Worked example
1
A 70 kg driver travels at 15 m/s and is brought to rest by a seat belt in 0.50 s.
2
Change in momentum = 70 × 15 = 1050 kg m/s.
3
Force = 1050 ÷ 0.50 = 2100 N.
4
If the stop took only 0.10 s (rigid car, no belt), the force would be 10 500 N, five times larger.
Part 4 of 4Key points about collisions
Momentum is conserved
- Momentum is conserved In any collision with no external forces the total momentum before equals the total after.
Equal and opposite forces
- Equal and opposite forces During the collision each object pushes on the other with forces equal in size and opposite in direction, for the same time.
Energy
- Energy In most real collisions, some kinetic energy is transferred to thermal energy, sound and deformation.
Key terms
| Term | Meaning |
|---|---|
| Collision | An event in which objects exert forces on each other over a very short time. |
| Crumple zone | A part of a vehicle designed to crush and absorb energy in a crash. |
| Airbag | A bag that inflates in a crash to cushion the occupant. |
| Impulse | The change in momentum of an object, equal to force × time. |
| Change in momentum | The final momentum minus the initial momentum. |
Avoid these mistakes
1
Saying safety features reduce the change in momentum. They do not; they increase the time, which reduces the force.
2
Stating that crumple zones make the car stronger. They are deliberately weaker so they crush.
3
Forgetting that the forces in a collision are equal and opposite, even if one object is much smaller.
4
Mixing up Δt with the speed of the car. Δt is the time taken to stop.
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