What is the PRIMARY reason a motorcycle helmet reduces head injury in a crash?

Helmet physics: OUTER SHELL (polycarbonate, fibreglass, or carbon fibre) distributes the point impact across a larger surface area, preventing skull penetration; ENERGY-ABSORBING LINER (expanded polystyrene, EPS foam) crushes progressively under impact — this controlled deformation slows the head's deceleration from crash speed to zero over a longer time and distance, dramatically reducing peak force on the brain; COMFORT LINER (cloth) — no safety function. Key principle: force = mass × acceleration (F=ma); if deceleration takes longer (helmet crushes), peak force is lower even with the same total mass and velocity change. This is why a single severe impact destroys the EPS liner — it has done its job and cannot be reused. Any helmeted crash = replace the helmet even if the exterior looks intact.