11.1 Distance and Displacement
Reading Focus
Key Concepts
What is needed to describe motion completely?- How are distance and displacement different?
- How do you add displacements?
Vocabulary
frame of reference
relative motion
distance
vector
resultant vector
Reading Strategy
Predicting Copy the table below and write a definition for frame of reference in your own words. After you read the section, compare your definition to the scientific definition and explain why the frame of reference is important.
| Frame of reference probably means | Frame of reference actually means |
|---|---|
|
a. |
b. |
Figure 1 You must choose a frame of reference to tell how fast the butterfly is moving.
Applying Concepts Identify a good frame of reference to use when describing the butterfly's motion.
On a spring day a butterfly flutters past. First it flies quickly, then slowly, and then it pauses to drink nectar from a flower. The butterfly's path involves a great deal of motion.
How fast is the butterfly moving? Is it flying toward the flower or away from it? These are the kinds of questions you must answer to describe the butterfly's motion. To describe motion, you must state the direction the object is moving as well as how fast the object is moving. You must also tell its location at a certain time.
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!
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?
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