Distance and Displacement

Cut to the Chase

Distance

Distance is one of those innate concepts that doesn't seem to require explanation. Nevertheless, a preliminary definition might be that distance is a measure of the interval between two locations. (This is not the final definition.) The distance is the answer to the question, "How far is it from this to that or between this and that?"

how far is it	possible answer	standard answer
earth to sun	1 astronomical unit	1.496 × 1011m
New York to Tokyo	6740 miles	1.084 × 107 m
heel to toe on my foot	11 inches	0.28 m

You get the idea. The odd thing is that sometimes we state distances as times. In casual conversation, it's often all right to state distances this way, but in most of physics this is unacceptable, except in Cosmology, where distances are sometimes expressed in terms of light-years. (This is the distance light travels in one year.)

Displacement

That being said, let me deconstruct the definition of distance I just gave you. If I walk around my desk, how far have I gone?

There are two ways to answer this question. On the one hand, there's the sum of the smaller motions that I made: two meters east, two meters south, two meters west; resulting in a total walk of six meters. On the other hand, the end point of my walk is two meters to the south of my starting point. So which answer is correct? Well, both. The question is ambiguous and depends on whether the questioner meant to ask for the distance or displacement.

Let's clarify by defining each of these words more precisely.

Distance is a scalar measure of the interval between two locations measured along the actua path connecting them.

Displacement is a vector measure of the interval between two locations measured along the shortest path connecting them.

Example

How far does the earth travel in one year? In terms of distance, quite far (the circumference of the earth's orbit is nearly one trillion meters), but in terms of displacement, not far at all (zero, actually). At the end of a year's time the earth is right back where it started from. It hasn't gone anywhere.

Distance and displacement are different quantities, but they are related. If you take the first example of the walk around the desk, it should be apparent that sometimes the distance is the same as the magnitude of the displacement. This is the case for any of the one meter segments but is not always the case for groups of segments. As I trace my steps completely around the desk the distance and displacement of my journey soon begin to diverge. The distance traveled increases uniformly, but the displacement fluctuates a bit and then returns to zero.

Distance (solid) and Displacement (dashed)

This artificial example shows that distance and displacement have the same size only when we consider small intervals. Since the displacement is measured along the shortest path between two points, its magnitude is always less than or equal to the distance.

How small is small? The answer to this question is, "It depends". There is no hard and fast rule that can be used to distinguish large from small. DNA is a large molecule, but you still can't see it without the aid of a microscope. Compact cars are small, but you couldn't fit one in your pocket. What is small in one context may be large in another. Calculus has developed a more formal way of dealing with the notion of smallness and that is through the use of limits. In the language of calculus the magnitude of displacement approaches distance as distance approaches zero.

Last, but not least, is the subject of symbols. How shall we distinguish between distance and displacement in writing. Well, some people do and some people don't and when they do, they don't all do it the same way. Although there is some degree of standardization in physics, when it comes to distance and displacement, it seems like nobody agrees.

What would be a good symbol for distance? Hmm, I don't know. How about d? Well, that's a fine symbol for us Anglophones, but what about the rest of the planet? (Actually, distance in French is spelled the same as it is in English, but pronounced differently, so there may be a reason to choose d after all.) In the current era, English is the dominant language of science, which means that many of our symbols are based on the English words used to describe the associated concept. Distance does not fall into this category. Still, if you want to use d to represent distance, how could I stop you?

All right then, how about x? Distance is a simple concept and x is a simple variable. Why not pair them up? Many textbooks do this, but this one will not. The variable x should be reserved for one-dimensional motion along a defined x-axis (or the x component of a more complex motion). Still, if you want to use x to represent distance, how could I stop you?

English is currently the dominant language of science, but this has not always been the case nor is there any reason to believe that it will stay this way forever. Latin was preeminent for a very long time, but it is little used today. Still, there are thousands of technical and not so technical words of Latin origin in use in the English language. Medicine, it seems, would be without vocabulary were it not for this "dead" tongue -- cardiac, referring to the heart; podiatry, the treatment of the feet; dentistry, the treatment of the teeth; etc. Examples are less common in physics, but they are there nonetheless. (There seem to be more Greek than Latin words in physics.)

Imagine some object traveling along an arbitrary path in front of an observer. Let the observer be located at the origin. The vector from the origin to the object points away from the observer much like the spokes of a wheel point away from its center. The Latin word for spoke is radius. For this reason, we will use r₀ (r nought) for the initial location, r for the location any time after that, and Δr (delta r) for the change in location -- the displacement. Unlike the spokes of a wheel, however, this radius is allowed to change.

Much more directly, but less poetically, the Latin word for distance is spatium. For this simple reason, we will use s₀ (s nought) for the initial location on a path, s for the location on the path any time after that, and Δs (delta s) for the space traversed going from one location to the other -- the distance.

If you think Latin deserves its reputation as a "dead tongue" then I can't force you to use these symbols, but I should warn you that their use is quite common. Old habits die hard. Use of spatium goes back to the first book on kinematics as we know it -- Galileo's Discourses on Two New Sciences in 1640.

Spatium transactum tempore longiori in eodem Motu aequabili maius esse spatio transacto tempore breuiori.

One important thing to notice in the diagram above is that the location of the observer does not really matter. You may think that the observer must be located at the origin, but this is not the case. It is merely convenient for the sake of illustration. If the observer were not at the origin, we could always move the origin to the observer. In addition, the x-axis need not be horizontal nor must the y-axis be vertical. No matter how you twist the coordinate system, the essence of the diagram remains unchanged. Distance and displacement are said to be isotropic, that is, they remain unchanged even if the coordinate system undergoes translation or rotation. All properly formulated physical laws must be isotropic.

units

The SI unit of distance and displacement is the meter [m].

A meter is a little bit longer than the distance between the tip of the nose to the end of the farthest finger on the outstretched hand of a typical adult. Originally defined as one ten thousandth the distance from the equator to the north pole (as measured through Paris); then the length of a precisely cut metal bar kept in a vault outside of Paris; then a certain number of wavelengths of a particular type of light -- the meter is now defined in terms of the speed of light. One meter is the distance light (or any other electromagnetic radiation of any wavelength) travels through a vacuum after 1/299,792,458 th of a second.

Multiples of the metre (like km for road distances) and divisions (like cm for paper sizes) are also commonly used in science.

There are also several natural units that are used in astronomy and space science.

Just the facts

Distance is the sum of the length travelled around the path of motion. It can also represent the net distance travelled between the start position and the enend position. This is called the displacement.

The distance is a scalar quantity while displacement is vector. Both are measured using the SI unit of metre [m].