Momentum, << moh MEHN tum, >> is a measure of the motion of an object. The momentum of a moving object equals its mass (quantity of matter) multiplied by its velocity (speed in a given direction).
Consider, for example, an automobile that weighs 1,000 kilograms (2,200 pounds), which means it has a mass of 1,000 kilograms. Suppose the car is moving north at 25 meters per second (about 55 miles per hour). The automobile’s momentum will be 25,000 kilogram meters per second toward the north. Now, consider a truck that weighs 5,000 kilograms (11,000 pounds). To have the same momentum as the car, the truck must travel northward at only 5 meters per second (about 11 miles per hour).
The English scientist and mathematician Isaac Newton discovered the laws of momentum. His findings appeared in Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy), a book published in 1687.
Newton’s laws include the law of conservation of momentum. According to this law, the momentum of an object will change only if a net force acts on it. A net force is a force that is not balanced by an opposing force.
In our example, several forces will tend to slow the automobile. These forces include the resistance of the surrounding air to the motion of the car and the friction force of the tires against the road. But the force created by the car’s engine will tend to make the vehicle go faster. If this force balances the forces tending to slow the car, the vehicle will travel at a constant speed. Our example does not include any force that would tend to change the automobile’s direction. Therefore, the vehicle’s velocity and momentum will also remain constant.
The law of conservation of momentum also applies to systems (interacting sets) of objects. Consider a system made up of two equally massive bumper cars. Suppose each car is going 4.5 meters per second (10 miles per hour), and the cars are moving directly toward each other. The two cars will have the same amount of momentum. However, the momentum of one will point in the direction opposite that of the other. The total momentum of the two cars will therefore be zero.
Suppose the cars collide head-on and bounce off each other at 4.0 meters per second (9 miles per hour). The collision will change both cars’ momentum. But, because no force from outside the system has acted on the cars, their total momentum remains zero.
