How do helmets work?

Question

can someone explain how helmets work using the laws of physics

Answer 1

Because they are rounded,the bullet has less chance to peneterate than to bounce off.

Answer 2

Helmet offers two different types of protection.

First of all, most helmets have an inner foam which does help cushion your head from the impact, and so prevent bruises. This is quite self explanatory, as having a softer inside makes it less painful when something drops on your head.

The second key important point of helmets, is that because they are hard, they spread the impact AROUND the entire helmet instead of having it focused at one point.

Imagine a brick dropping on your head.. if the sharp edge hits your skull, it will most definitely crack it. But with a helmet, it dissipates (or spreads) this impact around the entire helmet, so that you feel the entire helmet vibrate, instead of feeling all of the impact at one small point. With the foam inside the helmet, you can pretty much survive a brick falling on your head.

But if its something huge and too heavy, like a piano... a helmet can't do very much =P

Answer 3

Quite an interesting question. The helmet is used for two purposes. 1. To protect the head from high accelerations due to external impact forces and 2. to stop hard sharp objects that strike the helmet from penetrating to the skull.

The structure of the helmet provides a means of absorbing strain-energy, due to its stiffness. During an impact the kinetic energy of the impact is converted into this strain energy with the result than much of the energy of motion never gets to the head. The latter is supported by straps and softer materials inside the hard and stiff (but flexible) outer shell. Displacements of the shell will cause light pressure within these softer materials and so the head is not badly jarred.

When a hard or sharp object having relatively small kinetic energy (compared to the above sceanrio) strikes the helmet, its protective action is either by absorbing the energy in the same way as above, but to a more local degree, or it causes the striking object to slide and bounce off so that most of its energy is not converted at all.

The kinetic-energy/strain-energy convertion is by the following formula:

(M/2) x V^2 = k x z^2

Where M is the mass of the head and other non-elastic parts of the body involved in the collision (alternatively for the second kind of impact it is the mass of the striking object).

V is the speed of impact

k is the stiffness of the helmet in terms of the force needed to deform it to a specific degree associated with the impact

and z is the displacement of the outer surface of the helmet itself.

Then the force of impact is:  F = k x z which may be substituted into the first equation too.

Answer 4

Helmets are useful as safety gear to prevent injuries in an uncontrolled environment. If you can't prevent a crash or impact, but you know it will occur, a helmet can prevent or minimize injury to the head and brain.

Helmets designed to handle major crash energy generally contain a layer of crushable foam. When you crash and hit a hard surface, the foam part of a helmet crushes, controlling the crash energy and extending your head's stopping time by about six thousandths of a second (6 ms) to reduce the peak impact to the brain. Rotational forces and internal strains are likely to be reduced by the crushing.

Thicker foam is better, giving your head more room and milliseconds to stop. If the foam is 15mm thick it obviously has to stop you in half the distance of a 30mm thick foam. Basic laws of physics result in more force to the brain if the stopping distance is shorter, whatever the "miracle" foam may be. Less dense foam can be better as well, since it can crush in a lesser impact, but it has to be thicker in order to avoid crushing down and "bottoming out" in a harder impact. The ideal "rate sensitive" foam would tune itself for the impact, stiffening up for a hard one and yielding more in a more moderate hit.