MadSci Network: Physics

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

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 
your concern.

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