The laws of momentum and energy govern collisions between two objects (Physics). A collision occurring in an isolated system conserves the total momentum of the system and so the total momentum of the system after the collision is the same as it was before the collision. For instance, if a truck weighing 3000kg is traveling at a velocity of 20 m/s, it has a total momentum of 60,000 kg/m/s. If it collides with a stationary motor cycle weighing 1000 kg, with 0 m/s velocity and therefore 0 kg/m/s momentum, after the collision the motor cycle will have a velocity of 30 m/s and a momentum of 30,000 kg/m/s, and the truck will have a velocity of 10 m/s and a momentum of 30,000 kg/m/s.
In an inelastic collision, if the motor cycle is traveling at 20 m/s it will have a momentum of 20,000 kg/m/s before the collision (Physics/Inelastic). If it collides with a stationary truck weighing 3000 kg, after the collision the motor cycle will have a velocity of 5 m/s and a momentum of 5,000 kg/m/s and the truck will ave a velocity of 5 m/s and a momentum of 15,000 kg/m/s.
A motorcycle collision will be inelastic if the rider hits the ground or some other stationary object. If the rider hits a moving object, e.g. a moving car or truck, the collision will be elastic. During an elastic collision, kinetic energy is conserved, but during an inelastic collision, kinetic energy is converted to other forms of energy such as heat energy and sound energy (Physics/Inelastic).
Motor cycle helmets are designed to reduce the momentum of the head in a collision with another object e.g. a car, the ground. The total momentum in the collision between the helmet and the object with which it collides will be conserved, but the helmet is designed to absorb much of the energy generated so that the head is not subjected to the full impact of the collision, thus protecting the brain which would otherwise impact with the skull with the same momentum with ...