Two Ways to Describe How Far an Object Moves
Phred and Phoebe are physicists. Around Halloween, they observe a ghostly (but smiling!) head moving about. Each of them describes the motion in their own way.
The head starts at position 0 m, then stops at position 2 m.Phred says the face moved 2 m, and so does Phoebe. Do you agree with this description?
This time the head starts at 4 m, and ends up at 1 m.
Phred says the face moved 3 m, but Phoebe says it moved –3 m.
Phred and Phoebe used different descriptions of the same motion. Is one of them wrong? Explain your answer.
Notice that this time the disembodied head starts at –2 m, it goes to position 3 m, and ends up at position 1 m.
Phred says the face moved 7 m, but Phoebe says it moved +3 m.
Phred and Phoebe are both right, based on their definitions of the word “moved”.
What is Phred describing?
What is Phoebe describing?
You can read the picture this time.
Phred says the face moved 10 m.
How far would Phoebe say it moved?
You can read the picture this time.
How far would Phred say the head moved?
How far would Phoebe say it moved?
Suppose Phred said the head moved 6 m, and Phoebe said it moved –4 m.
Show this motion in the same style as above.
Are there other ways the movement could match this description?
Phred and Phoebe are each using a different way to describe how far the head moved.
One way to describe “how far” is to compare the starting position and the ending position. The displacement is how far the ending position is from the starting position, and the direction, using + for one way and – for the opposite direction.
Another way is to describe the distance, which is the total amount of space covered by the motion, including all the back-and-forth, and ignoring the direction.
- Which of the physicists was describing distance?
- Give an example of a situation in which distance is a useful way to describe motion.
- Which of the physicists was describing displacement?
- Give an example of a situation in which displacement is a useful way to describe motion.
A and B both say: 4 m
A says 6 m, B says –6 m
A says 3 m, B says –3 m
A says 7 m, B says 7 m
A says 10 m, B says 2 m