What physics concept is used when designing a motorcycle helmet to protect the head from injury

From the 1st Law of linear motion which states that when a body goes into motion, it will continue doing so until it is stopped by force. So the body of the rider keeps moving until it is been stopped and the stopping could be as a result of an impact of any part of the body which includes the head it an object. Also the 3rd Law of Motion will also be applicable because for every action, there will be equal and opposite reaction. The magnitude of the impact will be as a result of the force with which the crash took place as well.

When crashes take place the rider does not always experience a head impact square on with a solid obstruction. During a bike crash, your head comes in contact with the ground. The ground exerts a force that causes your head to stop moving. Often impact will be at an angle and may not be head first. It may be your shoulder will hit first, then your side, and then your head will receive a glancing blow against the ground as you slide.

The ground exerts so much force that it can stop our forward motion within seconds. Without your motorcycle helmet, your head experiences a huge amount of concentrated force during a crash.