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10.7 Safety First

GCSE Physics · AQA · Chapter 10: Force and Motion

A free GCSE Physics lesson for AQA (8463) from OBA School: how helmets, crash mats, seat belts, air bags, crumple zones and side impact bars reduce injury. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • (Higher tier) explain how cycle helmets, cushioned surfaces and crash mats reduce impact forces
  • (Higher tier) explain how seat belts and air bags reduce the force on people in a car crash
  • Describe how crumple zones, side impact bars and collapsible steering columns work, and why baby seats face backwards
  • Describe how thinking, braking and stopping distances change with speed
  • (Higher tier) use conservation of momentum to decide whether a vehicle was speeding before a collision

Key words

  • Impact time: The time taken for a moving object to be brought to rest in a collision.
  • Seat belt: A strap that holds a person in their seat; it stretches slightly and spreads the force across the chest and hips.
  • Air bag: A bag that inflates very quickly in a crash to cushion the head and chest.
  • Crumple zone: A part of the car body at the front or rear that folds up in a collision.
  • Side impact bar: A strong steel bar inside a car door that protects people in a side-on collision.
  • Collapsible steering column: A steering column that shortens in a crash so the driver’s chest is not hit by a rigid wheel.
  • Cushioned surface: A soft surface, such as rubber matting or a crash mat, that squashes when something lands on it.

The big idea: make the stop take longer (Higher tier)

In an accident, a person’s momentum must drop to zero. The change of momentum is fixed by their mass and speed. Using F = m Δv ÷ Δt, the force on them can only be made smaller by making the impact time longer. Spreading the force over a larger area also reduces the pressure on any one part of the body.

Key fact: Safety features do not reduce the change of momentum. They reduce the rate of change of momentum, which is the force.

Helmets, playground surfaces and crash mats (Higher tier)

  • Cycle and riding helmets have a hard outer shell that spreads the force and a crushable foam liner. The liner squashes, so the head stops over a longer time and the force on the skull is smaller. A helmet that has been in a crash must be replaced because the foam stays crushed.
  • Cushioned playground surfaces (rubber matting, bark chippings) squash when a child lands, increasing the impact time compared with concrete.
  • Crash mats in a gym do the same for gymnasts and high jumpers.

Worked example: a fall in the playground

A 25 kg child falls and hits the ground at 5.0 m/s. Change of momentum = 25 × 5.0 = 125 kg m/s.
Concrete, stopped in 0.010 s: F = 125 ÷ 0.010 = 12 500 N
Rubber surface, stopped in 0.10 s: F = 125 ÷ 0.10 = 1250 N

Answer: ten times the impact time gives one tenth of the force

Car safety features

  • Seat belts stop you being thrown forward into the windscreen or dashboard. The belt stretches slightly, so you stop over a longer time, and it spreads the force across your chest and hips.
  • Air bags inflate in a few hundredths of a second. They increase the time for your head and chest to stop and spread the force over a large area. They work together with the seat belt.
  • Crumple zones fold up, increasing the impact time for the whole car; the energy transferred in deforming them is not passed on to the people inside.
  • Side impact bars make the doors stronger so that less of the car is pushed in during a side-on crash, and they help to spread out the impact.
  • Collapsible steering columns shorten in a crash so the driver’s chest is not hit by a rigid steering wheel.
  • Rear-facing baby seats: a baby’s neck muscles are weak. In a sudden stop, a rear-facing seat pushes on the baby’s whole back and supports the head, spreading the force over a large area.

Worked example: seat belt and air bag (Higher tier)

A 70 kg passenger moving at 14 m/s is stopped by a seat belt and air bag in 0.15 s.
F = 70 × 14 ÷ 0.15 = 6500 N (2 s.f.)
Without them, hitting the dashboard would stop the passenger in about 0.010 s: F = 980 ÷ 0.010 = 98 000 N

Answer: the safety features reduce the force about 15 times

Stopping distances and speed

  • Thinking distance in m; 9 6.3; 18 12.6; 27 18.9
  • Braking distance in m; 9 6.0; 18 24.0; 27 54.0
  • Stopping distance in m; 9 12.3; 18 36.6; 27 72.9

For a driver with a reaction time of 0.7 s on a dry road: doubling the speed from 9 m/s to 18 m/s doubles the thinking distance (thinking distance ∝ speed) but makes the braking distance four times larger (braking distance ∝ speed²). This is why speed limits near schools are low.

Was the driver speeding? (Higher tier)

Accident investigators measure skid marks to find how fast vehicles were moving just after a collision. Then they use conservation of momentum to work backwards.

Worked example: a collision at a junction

A 900 kg car runs into a stationary 2100 kg van. The vehicles lock together. Skid marks show that they were moving at 8.0 m/s just after the impact. The speed limit is 22 m/s.
momentum after = (900 + 2100) × 8.0 = 24 000 kg m/s
momentum before = 900 × u = 24 000, so u = 26.7 m/s

Answer: about 27 m/s – the car was well over the 22 m/s limit

Common mistakes

  • Saying safety features “absorb the force”. They increase the impact time (and the area), so the force is smaller.
  • Saying a safety feature reduces the change of momentum. The change of momentum is the same; it happens more slowly.
  • Saying a seat belt stops you instantly. It is designed to stretch so that you stop over a longer time.
  • Explaining a helmet only as “soft”. Link the squashing to a longer impact time and a smaller rate of change of momentum.
  • Forgetting to add both masses when vehicles lock together in an accident calculation.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) How does a cycle helmet reduce the force on a rider’s head in a fall? (Choose from: Its foam liner squashes, so the head takes longer to stop, It reduces the change of momentum of the head, It makes the head stop more quickly, It increases the mass of the head.)

Answer: Its foam liner squashes, so the head takes longer to stop

Working: The liner crushes, increasing the impact time. The same change of momentum over a longer time means a smaller rate of change of momentum, so a smaller force.

Question 2. (Higher tier) True or false? An air bag reduces the change of momentum of a passenger in a crash.

Answer: False

Working: False. The change of momentum is the same. The air bag increases the time taken to stop and spreads the force over a larger area, so the force is smaller.

Question 3. (Higher tier) A 20 kg child falls and hits a rubber playground surface at 4.0 m/s. She stops in 0.080 s. Calculate the average force on her.

Answer: 1000 N

Working: Δp = 20 × 4.0 = 80 kg m/s. F = 80 ÷ 0.080 = 1000 N.

Related lessons

Continue in the full course

This lesson is section 10.7 (AGCPH1007) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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