### Re: What's the physics behind impact resistance?

Date: Fri May 26 18:24:11 2000
Posted By: Denni Windrim, Staff, science, Sylvan Learning Centre
Area of science: Physics
ID: 958873356.Ph
Message:
```
You've got part of the answer already - a big jelly jat will smuck both the
eggs =and= the rice cake. What happens with a small jelly jar and a rice
cake is energy transfer. One of the oldest awe-inspiring tricks is lying on
a bed of nails. It seems dangerous, but when a person's weight is spread out
over several hundred nails, no matter how sharp (within reason), the load on
each nail is actually quite small, and so the feat is not nearly as
perilous, or as painful, as it appears. To lie on a bed of one nail,
however, would be very painful and injurious. Part of the reason the eggs
don't break is that the rice cake transfers the energy of the falling jar
equally to the entire contact region between it and the eggs beneath,
considerably lessening the stress at any given contact point. There is a
second factor. If you pull an individual puffed rice grain from the cake and
aqueeze it, you will note that it is spongy. This sponginess also pulls
energy from the falling-jar equation by absorbing the kinetic energy in the
deformation of the rice cake instead of the deformation of the eggs -
essentially, what you've given your eggs is a high-impact bumper. This
principle is widely used in automotive design - by building front ends that
crumple (and hence absorb energy in their deformation), the deceleration of
the passenger compartment and the folks therein is reduced enough to lower
the likelihood of injury or death. If you used a plastic or glass plate
instead of a rice cake, you would note that the size of the jelly jar
necessary to break the eggs would be even smaller. Your eggs thank you for

```

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