Displacement or position?
[22 Jan 2007]
Ewen, a visitor to the Interactive Mathematics site, writes:
On your displacement-time graph, it is more correct to title the vertical axis “position”. From the vertical axis, you can then calculate the “displacement” by simply subtracting one position from another between any two times. It’s like labelling the horizontal axis “time interval” rather than just “time”. You’re not the first to make this mistake. I’ve seen it in a few textbooks. (But, they’re usual textbooks for lower grades, or lower levels of Physics. If that’s why you’ve done it, then I understand.)
I believe the term “displacement” is used correctly here. At the top of the page, the first graph correctly uses “distance” to describe the total length of the journey. For the graphs under the heading “Displacement-time graphs”, the axis description “displacement” is used correctly to indicate “how far from the origin we are at that time”.
In the introduction to the Kinematics chapter, I state:
“Distance” normally refers to the total distance an object moves during a particular journey.
“Displacement” refers to the distance from the starting point at a particular instant in time.
Wikipedia says displacement…
“…specifies the position of a point or a particle in reference to an origin”
I’m not sure that “position” adequately describes what we mean. If it was something like “position in km from the origin” maybe, but it would be better as “distance from the origin” which is really “displacement” anyway.
Anyone like to weigh in on this?