The momentum of an object is the mass of the object multiplied by the velocity of the object. The mass will often be measured in kilograms (kg) and the velocity, in meters per second (m/s), so the momentum will be measured in kilogram meters per second (kg m/s). Because velocity is a vector quantity, meaning that the direction is part of the quantity, momentum is also a vector. Just like the velocity, to completely specify the momentum of an object one must also give the direction.

A force multiplied by the length of time that the force acts is called the impulse. According to the impulse momentum theorem, the impulse acting on an object is equal to the change in the object's momentum. Notice the word change. The impulse is not equal to the object's momentum, but the amount the momentum changes. (This impulse momentum theorem...