So your head didn’t hit the stone of the road, I guess that helps.
But, you still hit your head on *something,* essentially. The inside of your helmet.
All that force is still in there, right?
In: Physics
* Force = Mass X Acceleration.
* We want to reduce the force your head experiences so according to physics, we can only reduce two things, the mass involved, and/or the acceleration.
* Well we can’t really reduce the mass of your head, or anything that might hit it.
* So the only left is to reduce the acceleration.
* But what is acceleration?
* It’s how quickly you change speed.
* When something hits your head, the damage is usually done because your head was moving and *very very* *quickly* it comes to a stop.
* So we need to increase the amount of time it takes to make your head stop.
* This is where the helmet comes in.
* It’s designed to cushion your head.
* Instead of hitting something hard that won’t move, it hits something soft that moves a little.
* This slows down your head just enough to decrease the force applied enough to prevent major injury.
Smack a table with your hand. What happens? It hurts because the table pushes back on your hand.
Now, smack a block of clay with your hand. What happens? It hurts less because the clay squishes and pushes back on your hand less hard.
A Styrofoam bike helmet is like the block of clay. When your head hits it the energy goes into squishing the Styrofoam rather than hurting your head.
In engineering and physics, the squishing is called a “deformation.” There’s a lot of physics around how deformations absorb forces but they are not usually covered by basic physics. Most basic physics focus on rigid body physics because it is simpler. Rigid is a thing that does not deform at all. (In real life, many materials deform, even if it’s just a little.)
A rigid helmet also distributes forces around your head so no one spot takes all the force, but others have already answered that part of the physics.
Damage is basically determined by the amount of energy that your head absorbs from the collision, and the energy depends a lot on the deceleration, and on the surface that’s impacted. So a lot of the stuff that’s employed to protect people functions on the basis of:
* spreading out the impact over a larger area (helmets, bullet-proof vests with metal plates in them, shields, etc.)
* making the collision take more time (air bags, crumple zones in cars, helmets, any sort of foam padding, etc.). You get a lot of damage if your head hits cement and goes from speed to 0 in a fraction of second, vs. your head hitting a foam matress and going from speed to 0 in maybe a second or two.
So as the others are saying, the helmet’s hard outer shell spreads out the force / energy of the impact over a larger surface, and the helmet’s inner foam padding lets your head sink into that foam, and that takes some time compared to a sudden impact against cement for example.
Force is based on the time deceleration takes. If the energy going into destroying the helm in won’t go into destroying your skull. It will slow down how fast your head stops moving.
The aim of your helmet is not to protect your head, but to protect your brain.
Your brain sits inside your skull floating in fluid. It shouldn’t bump into anything. Your skull is your natural helmet. It is domed, which is a great shape for dissipating force. It’s much harder to break a dome than it is to break a flat plane. A great way to test this out is to try and break an egg on the tip of the egg compared to the side. There will be a significant difference in force required.
When you get a brain injury, your brain smacks against the inside of your skull and is bruised. The aim is to stop that from happening.
A helmet is a domed shape that dissipates the initial force exactly like your skull would. Your skull then hits the inside of the helmet, but much less force is transferred from helmet to skull than from concrete to helmet. Your skull then dissipates the force even further and your brain stays floating in its fluid and never hits the inside of your skull.
A helmet that is properly fitted should be snug to the head. While the helmet is hard, it is much softer than the road or a rock. It is designed to break and crumple so your head does not. All that breakage and crumpling absorbs energy. Energy that your head did not absorb. Depending on how hard you hit your head you will likely still have a concussion though.
Explanation for a five year old because I don’t know the chemistry/biomechanics.
The inside of the helmet is made out of a very good shock absorption foam. This is the bit that saves lives.
The hard outer shell is more or less just a way to keep the foam there.
After a fall, the foam crumples and sacrifices itself to save you. After this one fall, the foam is unusable/too damaged and you have to get a new helmet. It also degrades in direct sunlight, but this isn’t an issue because the shell and inner fabric protect the foam.
When in doubt that a helmet’s foam is done after a fall, buy a new helmet.
Helmets help by spreading the impact out over a larger area, and by spreading the impact over a larger period of time. Cushioning stretches the impact over time so the force at any given moment is lessened. Think jumping onto a big cushion as opposed to the ground; you stop either way but slower hurts less. A firm outer covering spreads the impact over an area so the force on any given area is reduced. Think being pushed as opposed to being stabbed; stabbed hurts more because focused force will be enough to damage tissue.
Your head is still accelerating from moving to not-moving, yes. However, a helmet can slow that down by providing a cushion. The helmet stops first, then your head, by compressing the foam inside the helmet, slows down more gradually.
Remember that force is mass times acceleration. By reducing the acceleration that your head/brain is experiencing, you are exerting less force on the brain. Less force means that there’s less jostling and less damage, since most tissues can withstand some amount of force/strain/stress before they are damaged.
The inside of a helmet is cushioned, softening the blow. Also some helmets are like crumple zones for cars. They’re designed to absorb the impact and transfer that force into something else (in this case the structure of the helmet) instead of that force going into your head.
Also, motorcycle helmets are next level durable. If u hit the road and slide, that’s a few millimetres of metal scraped off the helmet instead of meat off your skull.
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