Designing for Safer Collisions
Aligned to HS-PS2-3 — Next Generation Science Standards.
What this lesson teaches
During a collision the change in momentum (impulse) is fixed by the crash, but impulse equals force times time (F·t = Δp). Stretching the collision over more time lowers the peak force. Crumple zones, airbags, and helmet foam all work by extending the stopping time.
Worked example
A 0.5 kg egg moving at 8 m/s must stop (Δp = 0.5 x 8 = 4 kg·m/s). Hitting concrete it stops in 0.01 s, so F = 4/0.01 = 400 N. Landing in foam that stops it in 0.2 s gives F = 4/0.2 = 20 N, twenty times gentler.
Practice questions
- State the impulse-momentum relationship and identify which quantity a safety device is trying to reduce.
- A crash test dummy (75 kg) decelerates from 15 m/s to 0. Compare the force if stopping time is 0.05 s versus 0.5 s.
- Sketch and describe a package to protect a dropped phone; explain each feature using impulse and stopping time.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Diseñar para Choques Más Seguros
Durante un choque, el cambio de momento (impulso) queda fijado por el impacto, pero el impulso es igual a la fuerza por el tiempo (F·t = Δp). Extender el choque durante más tiempo reduce la fuerza máxima. Las zonas de deformación, las bolsas de aire y la espuma del casco funcionan alargando el tiempo de frenado.
Ejemplo: Un huevo de 0.5 kg que se mueve a 8 m/s debe detenerse (Δp = 0.5 x 8 = 4 kg·m/s). Al golpear el concreto se detiene en 0.01 s, así que F = 4/0.01 = 400 N. Al caer en espuma que lo detiene en 0.2 s, F = 4/0.2 = 20 N, veinte veces más suave.
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