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1.3.4States of Matter

Momentum and Safety Features — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 27 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Section 1 of 5Safety Features

Each safety feature is designed to increase the time over which a passenger's momentum changes.

Crumple zone
  • Crumple zone A front section of a car that deforms in a collision, lengthening the time taken to stop.
Airbag
  • Airbag An inflatable cushion that slows the passenger's head and body gradually during a crash.
Seatbelt
  • Seatbelt A belt that stretches slightly, holding the passenger and slowing them over a longer time.
Padding and helmets
  • Padding and helmets Soft materials that compress on impact, increasing the stopping time for the body.

Section 2 of 5How the Features Work

In all cases the change in momentum is the same, but the time over which it happens is longer.

1
In a crash the passenger's momentum must fall to zero, whatever safety features are fitted.
2
A crumple zone deforms, so the car takes longer to stop and the force is reduced.
3
An airbag compresses as the passenger moves into it, extending the stopping time.
4
A seatbelt stretches a little, spreading the change in momentum over a longer time.

Section 3 of 5Force and Time

Force and Time
Same change
  • Same change The change in momentum is fixed by the speed before and after the impact.
Longer time
  • Longer time A longer time of impact gives a smaller force, since F = Δp ÷ t.
Shorter time
  • Shorter time A short impact time gives a very large force, which increases the risk of injury.
Safety design
  • Safety design Engineers aim to extend the stopping time as much as possible within the vehicle's structure.

Section 4 of 5Worked Example

Question

  • A 1000 kg car travelling at 20 m/s stops.
  • Compare the average force on the car when it stops in 0.2 s and in 0.8 s, taking the car's initial direction as positive.

  1. Change in momentum Δp = 1000 × (0 − 20) = −20 000 kg m/s, so the magnitude is 20 000 kg m/s.
  2. Stopping in 0.2 s: F = 20 000 ÷ 0.2 = 100 000 N.
  3. Stopping in 0.8 s: F = 20 000 ÷ 0.8 = 25 000 N.
  4. The longer stopping time reduces the force to one quarter of its value.

Answer Stopping in 0.8 s gives a force of 25 000 N, compared with 100 000 N when stopping in 0.2 s.

Section 5 of 5Exam Checklist & Tips

1.3.4 – Key Points for the Exam

  • Crumple zone A deformable part of a vehicle that increases the time taken to stop in a collision.
  • Airbag A cushion that inflates to increase the impact time for a passenger.
  • Rate of change of momentum The change in momentum divided by the time over which it takes place.
  • Impact time The time over which a collision or stopping process takes place.

Common Mistakes

  • Saying safety features reduce the change in momentum; they reduce the force by increasing the time.
  • Stating that a larger time of impact increases the force; a longer time reduces the force.
  • Forgetting that the passenger's momentum must still fall to zero in a crash.
  • Writing that airbags stop the passenger instantly; they increase the time, but the stop still takes time.

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