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Distance


Alignments to Content Standards: 8.F.B.5

Task

Below are two graphs that look the same. Note that the first graph shows the speed of a car as a function of time and the second graph shows the distance of a different car from home as a function of time. Describe what someone who observes the car’s movement would see in each case. 1_00dcbb3d30aaaa5081ea2862b9cb4d6f

IM Commentary

In this task students interpret two graphs that look the same but show very different quantities. The first graph gives information about how fast a car is moving while the second graph gives information about the position of the car. This problem works well to generate a class or small group discussion. Students learn that graphs tell stories and have to be interpreted by carefully thinking about the quantities shown.

Solution

For the distance function, output values tell us how far from home the car is. For a speed function, output values tell us how fast the car is moving. Since we don’t have scales on either axis, we can’t talk about specific values of time, distance and speed, but we can make qualitative statements about distance and speed.

Distance Graph: The car starts its trip at home. It moves away from home at a constant rate. When the graph is horizontal, the car's distance from home is not changing, which probably means it has come to a stop for awhile.* Then the car moves farther away from home before turning around and coming back home. After staying at home for a time, the car moves away from home at a constant rate. It comes to a stop for a while* before coming back home.

*If the distance from home is not changing, it is also possible that the car is driving along a circle with the driver's home at the center.

Speed Graph: The car starts at rest and speeds up at a constant rate. When the graph becomes a horizontal line, the car is maintaining its speed for a while before speeding up for a short time and then quickly slowing down until it comes to a complete stop. It stays stationary for a little while where the graph is on the horizontal axis. Then the car speeds up, goes at a constant speed for a while and then slows down and comes to a complete stop.

Cam says:

over 3 years

Indeed. Thanks!

kassymov says:

over 3 years

"The first graph gives information about how fast a car is moving while the second graph gives information about the position of the car."

The other way around?

Jason says:

over 4 years

1) The word "velocity" in this task should be changed everywhere to "speed." (Velocity is a vector.)

2) In the asterisked text, I would delete the words "although this doesn't seem very likely." I think the word "probably" up in the main body of text already addresses the likelihood question and gives enough editorial.

3) A couple of edits to the explanation of the distance graph:

  • "It moves away from home at a constant speed." This strikes me as being most unlikely... What we do know from the graph is that the car moves away from home at a constant rate.

  • "After staying at home for a time, the car moves away from home at a constant speed." [[Same issue.]]

Kristin says:

over 4 years

Agree with all your points--the task should be fixed now.