SNHS Physics Blog 7: Momentum

Yunhui Shim

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Each moving object has two identifiable characteristics. They are mass and velocity. When skating on a skateboard and approached with a ball that is thrown by a friend, that ball when caught will make you move with a certain speed. However, when a friend throws a lightweight but fast moving ball versus a slow but heavy ball, it is quite astonishing that both of those two very different throws of objects have quite similar impacts. Balls have the same effect if they have the same momentum, and tis momentum is defined to be the mass times the velocity. The symbol of p is momentum, and the units are kg(m/s). Whenever, an object’s mass of velocity changes, the ball is to face changes in its momentum. P is usually distinguished as the linear momentum, and this is so that we could distinguish it from the angular momentum, which is associated with a rotating object.

The vectors for momentum and velocity are related. In many situations, the magnitude of the momentum of the object, rather than the direction is of interest. However, momentum is a vector, and the momentum vector follows the direction of the velocity vector. A momentum of direction is important, and to put it into perspective, we can consider an experiment. A beanbag bear and a rubber ball with the same mass of m, and the same downward speed of v hits the floor. The bean bag when coming to rest has a change in momentum of mv upward, but the rubber ball when coming to rest has a change in momentum of 2mv, upward counting that it has bounced upward with a speed of v. To analyze this data, imagine a coordinate plane. Before the beanbag was thrown, there was a negative momentum, and in order to increase this momentum from a negative value to zero, there needed to have been a positive, upward momentum. The same goes for the rubber ball, as when we analyze the vector nature of momentum, it is possible to realize that the momentum had to first be increased from a negative value to zero, and then it had increased once more from zero to a value, and thus the total momentum was twice of the beanbags’ momentum. It is important to realize that the ball’s momentum did change, due to the fact that the ball’s direction of motion changed.

The systems that are analyzed in physics will most often contain more than one moving object. This can be derived by the total momentum of a system of objects, or the vector sum of the momentum of the individual objects. The total momentum is a vector sum, and these momentums of objects headed in opposite directions can partially or totally cancel one another. Therefore, the total momentum of objects may be positive, negative, or zero.