Physics / Unit 6: Momentum and Collisions
6.3 Impulse and Safety
Notes
For a fixed change in momentum, $\displaystyle F = \frac{\Delta p}{\Delta t}$: a longer stopping time means a smaller average force.
- Air bags, seat belts, crumple zones, helmets and padded mats all increase the stopping time.
- Bending your knees when you land spreads the stop over more time.
- In sports, "following through" makes the contact time longer, so the ball gets a bigger impulse.
- Example: a 70 kg person stopping from $15 \text{ m/s}$ in 0.1 s feels $\displaystyle \frac{1050}{0.1} = 10500 \text{ N}$; stopping in 0.5 s, only $2100 \text{ N}$.
Practice
How does an air bag reduce injury in a crash?
The momentum change is the same; spreading it over more time lowers the force.
An egg is dropped onto a pillow and does not break. An identical egg dropped from the same height onto concrete breaks. Why?
Both eggs have the same change in momentum; the pillow increases $\Delta t$.
A 60 kg stunt performer falling at $10 \text{ m/s}$ lands on an air cushion and stops in 0.6 s. What average force does the cushion exert?
$\displaystyle F = \frac{60 \times 10}{0.6} = 1000 \text{ N}$.
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