Helmet Safety - Measuring Force of Impact

[jdilla310]

My 12 year old daughter is conducting an experiment that includes testing various sporting helmets to determine which will protect your head best from a concussion. She is planning on dropping a 10 pound weight (approximate weight of human head) onto each side of the helmet. How can she determine the force impact? We don't have any measuring devices, so we are hoping to use simple formulas. Can anyone help?

 

[A.T.]

How can she determine the force impact?

Search "impact force" on this forums or in Google. Short answer: There is no simple formula for the peak force that occurs. At best you can use a smart phone and try to measure the peak acceleration. But I doubt you will get meaningful results for helmet safety with home methods.

 

[eigenperson]

This is not so easy!

One low-tech method for estimating forces is to put a block of putty between the helmet and a hard object. Measure the thickness of the putty. Then, after the force is applied to the helmet, the putty will be squashed and you can measure the new thickness of the putty.

Afterwards, you can determine what force was applied by placing heavy weights on top of a similar block of putty until it is squashed by the same amount. At that point, you assume that the same force was required in both cases, so the force of impact must have been equal to the weight.

This approach isn't very accurate, but it's the only thing I can think of that doesn't require a high-speed camera or an accelerometer.

 

[sophiecentaur]

I'm afraid there isn't a simple formula which deals with this sort of problem. There are a lot of parameters involved.
There are two aspects to the protection that a helmet needs to give. It needs to stop an object from getting to and penetrating your skull (obvious). But it also needs to spread the time of the impact as much as possible so that the force, slowing the projectile, can be lower. Helmets need a strong shell but they also need absorbent material to dissipate the energy slowly. A short duration, high value force can damage the brain inside (or neck), even though the skull is not penetrated.
The term "force of impact" is not a good one, although it is very frequently used in an attempt to explain / predict the likely damage. PF is full of questions about car accident, based on this misapprehension that it's all to do with force.

The 'putty' experiment could give some idea of comparative effectiveness / ranking of the helmets, although it would be difficult to 'calibrate' (meaningfully) the method against a static force, as suggested above. OF course, this could be a pretty expensive experiment because any helmet that is used in a meaningful (realistic) test (i.e. involving high stress) would not be suitable for future use. Most protective gear has a label which implies it should only be used once 'in anger'.