Читать книгу Physics I For Dummies - Steven Holzner - Страница 45

You find displacement by finding the distance between an object’s initial position and its final position. Say, for example, that you have a fine new golf ball that’s prone to rolling around, as shown in Figure 3-1. This particular golf ball likes to roll around on top of a large measuring stick. You place the golf ball at the 0 position on the measuring stick, as you see in Figure 3-1, diagram A.

FIGURE 3-1: Examining displacement with a golf ball.

The golf ball rolls over to a new point, 3 meters to the right, as you see in Figure 3-1, diagram B. The golf ball has moved, so displacement has taken place. In this case, the displacement is just 3 meters to the right. Its initial position was 0 meters, and its final position is at +3 meters. The displacement is 3 meters.

In physics terms, you often see displacement referred to as the variable s (don’t ask me why).

Scientists, being who they are, like to go into even more detail. You often see the term s_i, which describes initial position, (the i stands for initial). And you may see the term s_f used to describe final position.

In these terms, moving from diagram A to diagram B in Figure 3-1, s_i is at the 0-meter mark and s_f is at +3 meters. The displacement, s, equals the final position minus the initial position:

Displacements don’t have to be positive; they can be zero or negative as well. If the positive direction is to the right, then a negative displacement means that the object has moved to the left.

In diagram C, the restless golf ball has moved to a new location, which is measured as –4 meters on the measuring stick. The displacement is given by the difference between the initial and final position. If you want to know the displacement of the ball from its position in diagram B, take the initial position of the ball to be ; then the displacement is given by

When working on physics problems, you can choose to place the origin of your position-measuring system wherever is convenient. The measurement of the position of an object depends on where you choose to place your origin; however, displacement from an initial position s_i to a final position s_f does not depend on the position of the origin because the displacement depends only on the difference between the positions, not the positions themselves.