11.2 Speed and Velocity
Reading Focus
Key Concepts
How are instantaneous speed and average speed different?- How can you find the speed from a distance-time graph?
- How are speed and velocity different?
- How do velocities add?
Vocabulary
speed
average speed
instantaneous speed
velocity
Reading Strategy
Monitoring Your Understanding After you have finished reading this section, copy the table below. Identify several things you have learned that are relevant to your life. Explain why they are relevant to you.
What Is Relevant |
Why It Is Relevant |
|---|---|
a. ____?______ | b. ____?______ |
c. ____?______ | d. ____?______ |
e. ____?______ | f. ____?______ |
Figure 5 The speed of an in-line skater is usually described in meters per second. The speed of a car is usually described in kilometers per hour.
Look out a window for a few minutes, and you will see things in motion. Some things are moving slowly. Perhaps you see a leaf floating through the air. Other things, such as a car or a bird, are moving fast. The growth rate of trees and grass is so slow that their motion cannot be detected with the unaided eye. The differences among these types of motion can be described in terms of speed.
Speed
To describe the speed of a car, you might say it is moving at 45 kilometers per hour. Speed is the ratio of the distance an object moves to the amount of time the object moves. The SI unit of speed is meters per second (m/s). However, just as with distances, you need to choose units that make the most sense for the motion you are describing. The inline skater in Figure 5 may travel 2 meters in one second. The speed would be expressed as 2 m/s. A car might travel 80 kilometers in one hour. Its speed would be expressed as 80 km/h.
Two ways to express the speed of an object are average speed and instantaneous speed. Average speed is computed for the entire duration of a trip, and instantaneous speed is measured at a particular instant. In different situations, either one or both of these measurements may be a useful way to describe speed.
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