Moving objects have momentum. The more momentum an object has, the harder it is to change its direction. To calculate momentum you need to use the following formula:
momentum (kg m/s) = mass (kg) × velocity (m/s)
Total momentum stays the same in explosions and collisions, as long as no external forces act on the objects involved. When a car travels, everything in it has momentum; during a crash, forces act on passengers in the car and this can cause injury. If the time taken to change the momentum is increased, the forces on the bodies of passengers reduce. This is the role of seat belts, crumble zones and air bags.
To calculate the force involved in changing momentum, you can use the following equation: