In the case of a vehicle collision, the closed system is the two vehicles and the passengers. Before the collision, the total momentum of the system is equal to the sum of the momenta of the two vehicles. After the collision, the total momentum of the system is equal to the sum of the momenta of the two vehicles and the passengers.
Since the total momentum of the system is the same before and after the collision, it follows that the change in the momentum of one vehicle must be equal to the change in the momentum of the other vehicle. This means that the two vehicles will experience equal and opposite changes in velocity.
In other words, the vehicle that was hit will experience a decrease in velocity, while the vehicle that did the hitting will experience an increase in velocity. The amount of change in velocity will depend on the masses of the two vehicles and their relative velocities before the collision.
Momentum conservation plays an important role in vehicle safety. It helps to explain why it is important to wear seat belts and why airbags are so effective in reducing injuries in car accidents.
