11.1  Distance and Displacement (continued)

Choosing a Frame of Reference

How fast is the butterfly in Figure 1 moving? Remember that the butterfly is moving on Earth, but Earth itself is moving as it spins on its axis and revolves around the sun. If you consider this motion, the butterfly is moving very, very fast!

For: Links on comparing frames of reference

Figure 1  You must choose a frame of reference to tell how fast the butterfly is moving.

  To describe motion accurately and completely, a frame of reference is necessary. The necessary ingredient of a description of motion—a frame of reference—is a system of objects that are not moving with respect to one another. The answer to “How fast is the butterfly moving?” depends on which frame of reference you use to measure motion. How do you decide which frame of reference to use when describing the butterfly's movement?

How Fast Are You Moving? How fast are the train passengers in Figure 2 moving? There are many correct answers because their motion is relative. This means it depends on the frame of reference you choose to measure their motion. Relative motion is movement in relation to a frame of reference. For example, as the train moves past a platform, people standing on the platform will see those on the train speeding by. But when the people on the train look at one another, they don't seem to be moving at all.

Figure 2  To someone riding on a speeding train, others on the train don't seem to be moving.

Which Frame Should You Choose? When you sit on a train and look out a window, a treetop may help you see how fast you are moving relative to the ground. But suppose you get up and walk toward the rear of the train. Looking at a seat or the floor may tell you how fast you are walking relative to the train. However, it doesn't tell you how fast you are moving relative to the ground outside. Choosing a meaningful frame of reference allows you to describe motion in a clear and relevant manner.