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# Physics and Speed Help (page 2)

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By McGraw-Hill Professional
Updated on Sep 4, 2011

## Speed Practice Problems

Fig. 7.5 . Illustration for Problems 1 through 5.

#### Problem 1

Look at the graph of Fig. 7-5 . Curve A is a straight line. What is the instantaneous speed V inst at t = 5 seconds?

The speed depicted by curve A is constant; you can tell because the curve is a straight line. The number of meters per second does not change throughout the time span shown. In 10 seconds, the object travels 20 meters; that’s 20 m/10 s = 2 m/s. Therefore, the speed at t = 5 s is V inst = 2 m/s.

#### Problem 2

What is the average speed v avg of the object denoted by curve A in Fig. 7-5 during the time span from t = 3 s to t = 7 s?

#### Solution 2

Because the curve is a straight line, the speed is constant; we already know that it is 2 m/s. Therefore, V avg = 2 m/s between any two points in time shown in the graph.

#### Problem 3

Examine curve B in Fig. 7-5 . What can be said about the instantaneous speed of the object whose motion is described by this curve?

#### Solution 3

The object starts out moving relatively fast, and the instantaneous speed decreases with the passage of time.

#### Problem 4

Use visual approximation in the graph of Fig. 7-5 . At what time fis the instantaneous speed v inst of the object described by curve B equal to 2 m/s?

#### Solution 4

Take a ruler and find a straight line tangent to curve B whose slope is the same as that of curve A . That is, find the straight line parallel to line A that is tangent to curve B . Then locate the point on curve B where the line touches curve B . Finally, draw a line straight down, parallel to the displacement ( q ) axis, until it intersects the time ( t ) axis. Read the value off the t axis. In this example, it appears to be approximately t = 3.2 s.

#### Problem 5

Use visual approximation in the graph of Fig. 7-5 . Consider the object whose motion is described by curve B . At the point in time t where the instantaneous speed v inst is 2 m/s, how far has the object traveled?

#### Solution 5

Locate the same point that you found in Problem 7-7, corresponding to the tangent point of curve B and the line parallel to curve A . Draw a horizontal line to the left until it intersects the displacement ( q ) axis. Read the value off the q axis. In this example, it looks like it’s about q = 11 m.

Practice problems of these concepts can be found at: Mass, Force, And Motion Practice Test

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