Читать книгу Physics I For Dummies - Steven Holzner - Страница 49

There’s more to the story of motion than just the actual movement. When displacement takes place, it happens in a certain amount of time. You may already know that speed is distance traveled per a certain amount of time:

For example, if you travel distance s in a time t, your speed, v, is

The variable v really stands for velocity, but true velocity also has a direction associated with it, whereas speed does not. For that reason, velocity is a vector (you usually see the velocity vector represented as v or . Vectors have both a magnitude (size) and a direction, so with velocity, you know not only how fast you’re going but also in what direction. Speed is only a magnitude (if you have a certain velocity vector, in fact, the speed is the magnitude of that vector), so you see it represented by the term v (not in bold). You can read more about velocity and displacement as vectors in Chapter 4.

Just as you can measure displacement, you can measure the difference in time from the beginning to the end of the motion, and you usually see it written like this: . Technically speaking (physicists love to speak technically), velocity is the change in position (displacement) divided by the change in time, so you can also represent it like this, if, say, you’re moving along the x-axis:

Speed can take many forms, which you find out about in the following sections.