We use the terms momentum and impulse in daily life, but they have more specific meanings when used in physics. Nonetheless, we can understand the ideas without proper definitions. If a speeding car comes to a rapid stop, we understand that the people in the car have a forward momentum: after the car stops, they continue to go forward. We usually use the word impact to refer to a result: cars have a negative impact on the environment, the airbag can impact us by saving our lives. In physics, however, impact refers to the time over which a force is applied. Airbags do save lives, but they do so by reducing the impact of the passenger hitting the wheel or dash, by increasing the time over which the force of the crash affects the passenger.
The momentum of an object, in physics terms, is mass multiplied by velocity. The impact of an object in a collision is the force it applies multiplied by the amount of time that impact is applied for. Whether movement is in one or two dimensions, the momentum of objects in a closed system is always conserved. The equations for this relationship were presented in this chapter.