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MassachusettsPhysicsSyllabus dot point

How do safety devices like airbags and crumple zones use the physics of impulse to protect people in a crash?

Apply science and engineering ideas to explain how a device that extends the time of a collision reduces the force on an object, and evaluate a safety design (MA STE Introductory Physics, HS-PS2-3).

A standard-level answer on crash safety and engineering design for the Massachusetts High School Introductory Physics MCAS: how extending the collision time reduces force, how airbags and crumple zones work, and how to evaluate a safety design under HS-PS2-3.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. The core idea: extend the time, reduce the force
  3. How real safety devices work
  4. Evaluating a safety design
  5. Try this

What this topic is asking

This is the engineering-design standard of the momentum module, HS-PS2-3, and it is a favorite on the Massachusetts Introductory Physics MCAS because it connects physics to real safety. You must explain how a device that extends the time of a collision reduces the force on a person, name the devices that do this, and evaluate a safety design. The reasoning rests entirely on impulse: a fixed change in momentum spread over a longer time needs a smaller force. This is the practice of designing solutions.

The core idea: extend the time, reduce the force

The whole topic comes from one relationship between impulse and momentum. A given change in momentum can be produced by a big force acting briefly or a small force acting for longer:

  • Stop suddenly (short time): the force is large.
  • Stop gradually (long time): the force is small.

Crucially, the change in momentum is the same in both cases, because the person still goes from their initial speed to zero. The design lever is the time: anything that makes the stop take longer reduces the peak force the body feels.

How real safety devices work

Each common device is an application of this single principle:

  • Crumple zones. The front and rear of a car are built to crush in a crash. The crushing extends the time the car takes to stop, so the deceleration, and the force on the passengers, is reduced.
  • Airbags. An airbag inflates and then deflates as the person presses into it, extending the time over which the head and chest stop, and spreading the force over a larger area.
  • Seatbelts. A seatbelt holds the passenger with the car and is designed to stretch slightly, increasing the stopping time compared with hitting the dashboard.
  • Helmet and mat padding. Foam compresses during an impact, lengthening the stopping time for a head or a falling gymnast and cutting the peak force.

In each case the design increases the collision time, and through impulse that reduces the force.

Evaluating a safety design

HS-PS2-3 asks you to evaluate a design, not just describe it. A strong evaluation:

  1. Identifies that the device extends the stopping time.
  2. Explains that, for the same change in momentum, a longer time gives a smaller force.
  3. Notes any trade-offs (an airbag must inflate fast enough but not too hard; a crumple zone protects people but damages the car).

A weaker answer just says the device is "soft" or "absorbs the shock" without the time-and-force reasoning the standard wants.

Try this

Q1. State how an airbag reduces the force on a person in a crash. [2]

  • Cue. It extends the time over which the person stops; for the same change in momentum, a longer time means a smaller force.

Q2. A passenger's change in momentum is the same with or without a seatbelt. What does the seatbelt change? [1]

  • Cue. The stopping time (it lengthens it), and so the force (it reduces it).

Exam-style practice questions

Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

MA Physics MCAS (style)3 marksAn engineer is designing a car to protect passengers in a crash. (a) Explain how a crumple zone reduces the force on the passengers, in terms of impulse. (b) State one other design feature that uses the same principle. (c) State the quantity that must change by the same amount whether the crash is sudden or gradual.
Show worked answer →

A 3-point engineering-design item (HS-PS2-3) built on impulse.

(a) Up to 2 points: a crumple zone crushes during the crash, which extends the time over which the car (and passengers) come to a stop; for the same change in momentum, a longer time means a smaller force, so the passengers feel less force (1 point for longer time, 1 point for smaller force).
(b) 1 point: an airbag, a seatbelt with some give, a padded dashboard, or a helmet liner (any device that extends the stopping time).
(c) The change in momentum (the passengers must still go from their initial speed to zero, so the momentum change is the same; only the time, and so the force, differs). Markers reward the same change in momentum.

MA Physics MCAS (style)2 marksA gymnast lands on a thick foam mat instead of a hard floor. (a) Explain why the mat reduces the force on the gymnast. (b) State whether the gymnast's change in momentum is different on the mat than on the floor.
Show worked answer →

A 2-point item applying impulse to a safety design.

(a) 1 point: the foam mat increases the time over which the gymnast stops; for the same change in momentum, a longer stopping time means a smaller force, so the impact force is reduced.
(b) 1 point: no; the change in momentum is the same (the gymnast goes from the same landing speed to zero either way). Only the stopping time, and therefore the force, changes. Markers reward "same change in momentum."

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