# A couple of physics questions on helmets?

A couple of physics questions on helmets!?

1. Why is the impact force reduced for a helmet with a soft interior vs.a hard interior?

2. How is the safety reduced if the helmet does not fir properly?

3. Once a helmet has been involved in a collision, it should be replaced. why?

4. What sort of collision will be caused if a helmet collides with an objects? will it be an elastic, inelastic, or a completely inelastic collision? and why?

1. Some of the impact energy is absorbed by the deformation of well chosen material (ideally chosen so that energy of deformation equals the energy of an average collision). But a helmet also has to be comfortable if anyone is going to buy it.

2. Impact has to be distributed evenly across the head surface, so as to avoid pressure points where injury would first occur.

3. Because it has deformed permanently, due to having absorbed impact energy. Cracking a helmet takes a lot of energy. This is energy that will be absorbed by the skull instead when the next collision takes place.

4. Inelastic. Never either extreme in practice, because there will at least be noise and a little bouncing back (unless an object gets stuck in it).

Incidentally, I have experienced cracking a biking helmet, and I am grateful for being able to think about it today.

mgb_phys
Homework Helper

You can also think of a helmet reducing the rate that your head slows down.
If your head or a hard helmet hit the ground your head would stop suddenly - and so from f=ma you would experience a large force. By squashing, the outside of the helmet stops suddenly but the inside and your head can continue moving as the helmet crushes and so slow more gently. This is why even a rubber helmet that didn't permanently crush would still offer some protection.

Another reason to discard a helmet after a crash is that even if you cannot see any damage the insides may be crushed and so won't protect you next time. Finally for hard shell (motorcycle) helmets the plastics in the shell become weaker as bonds are broken down by sunlight, they should be replaced every few years - depending on the type of plastic.

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Ranger Mike
Gold Member

http://www.smf.org/

Snell Foundation has tested helmets since 1957..http://www.smf.org/ good info on brain buckets...

good place to start...if you have a hissy fit and throw your skid lid..you may have damaged it even if it is not apparent. Take good care of it..we have to replace our belts every three years and helmet every 5 years,,regardless of wear, accidents etc...

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