There are a lot of things we acquire when we turn 18: the right to vote, the ability to purchase lottery tickets, etc. We also gain the right to bicycle without a helmet. Whether or not this is good for us, many young Americans choose to embrace this freedom. Most don’t understand how helmets even serve to protect us from injury, but at the heart of the matter, physics explains how something so simple can prevent so much damage.
When we crash our bikes, the amount of damage we receive boils down to two main things: how fast we stop and how concentrated the force is.
“During a bike crash your head comes in contact with the ground and … the ground exerts forces that cause your head to stop moving,” said David Webb, a physics lecturer at UC Davis, in an email interview.
The ground exerts so much force that it can stop our forward motion within seconds. Without helmets, our heads then experience an incredible amount of concentrated force when we crash. This is the same as when we collide with anything. Replace the ground with another person, for example, and you can begin to understand why concussions are so common in sports like hockey and football.
This event where an opposing force stops the motion of another object or person is due to something called the law of conservation of energy. It states that energy can neither be created nor destroyed; therefore, when we slam to a stop, our kinetic energy (the energy of our motion on the bicycle) simply changes form. It interacts with the force the ground is exerting, and the kinetic energy then becomes a part of the force that hurts us.
Here’s where helmets come in. While they may not seem substantial enough to protect us, they are designed very efficiently.
“The outside of the helmet stops nearly instantly when it hits the ground but your head keeps moving within the outer shell of the helmet as it stops because of the padding as well as how the helmet holds your head. Because your head takes longer to stop, the forces exerted by the helmet are smaller than they would be if you didn’t have a helmet … A second thing that the helmet does is distribute the force it exerts over a wide area of your head so that your skull is less likely to be broken,” Webb said.
The shape and makeup of the helmet play the biggest role in protection. The foam in the helmet compresses, which allows your head to come to a stop much slower than it otherwise would have. This drastically reduces the acceleration. Since force is equal to mass times acceleration, it drastically reduces the force.
Furthermore, the arc-like shape of the average helmet distributes the force along its entirety, rather than just at the point of impact. Think of it like walking on grass in a pair of sneakers versus in a pair of high heels. Although your weight is the same in both, the high heels concentrate the force that your body exerts on the ground far more. This is why heels can sink into an otherwise tough lawn. In sneakers, on the other hand, you’re normally fine. Force from a collision can be similarly dispersed.
Although helmets for football don’t look like the ones cyclists use, nearly all helmets share the same basic design. There are still differences that target the specific demand of each situation. For example, hockey helmets must protect the wearer from any number of dangers.
Sam Lehr, a second-year history major, plays hockey for UC Davis.
“In a sport that is as fast-paced and as physical as hockey, helmets are essential to protect from the multitude of things that could cause damage to the head, including pucks, other players and the rink itself. I have seen head injuries and [I] myself have taken some nasty hits where my head was either the principal point of contact or ended up hitting something else hard. If not for the helmet I was wearing, the injuries I might have sustained could have been very serious. Instead I am usually… able to get right back into the play,” Lehr said.
Not everyone is lucky enough to escape from collisions without concussions, however. Amy Chyan, a second-year neurobiology, physiology and behavior major, experienced her first concussion last year when a bike accident without a helmet flipped her over her handlebars.
“I felt myself flip over as my face smashed into the pavement. I banana peeled and my bike kind of hit me over the head and crashed on top of me. I was twisted in my bike … I felt a lot of pain everywhere, and my head had really sharp throbbing pain. Biking home I was really dizzy and lightheaded,” Chyan said.
Chyan went to the Student Health and Wellness Center the next day for multiple tests and ultimately was sent to the local hospital for a CT scan to check for internal bleeding. She missed the rest of her classes that day and a few throughout the week, as she felt exhausted and was plagued by a near-constant headache.
While they may not seem very important, helmets do an incredible job in protecting us from harm. Whether it be from an opposing player on the ice rink or the unforgiving asphalt, many forces are strong enough to bring our motion to a complete and sudden stop. Helmets can take that force and minimize it by delocalizing it on our heads. They also give our skulls more time to stop as the foam condenses. Physics is always at play when motion is involved. In this case, it can explain the difference between a bad hit from a puck and a hospital stay.