Class content > The Main Question: Motion > Kinematics
Prerequisites
Length vs. position
While you've all used graphs and coordinate systems in your math classes, to describe motion we have to take the additional step of tying the coordinate system to the physical world. The twoaxis graph in math (an "xy plot") is a mathematical structure that allows us to use the tools of both geometry and algebra. But in tying it to the physical world, we are doing more. We are making a mathematical model of something in the physical world  something nontrivial, both conceptually and in deciding how much of the math we get to legitimately use. Let's go through the process carefully.
We've talked about how we can assign a number to a length using an operational definition by comparing a standard to the length we want to quantify. This isn't good enough for describing location. Consider the following story.
A fisherman went out in the early morning along the river to fish. At the first spot the fish weren't biting at all, so after about half an hour he moved to another spot. Still nothing. So he moved again. The sun was getting high now, so he was getting concerned that he had missed the best time of day. But at his third spot the fish were biting like crazy and he pulled in a satisfactory haul. When he was done, he wanted to remember this spot. Fortunately, he had remembered to bring a can of paint and a brush, so he painted an "X" on the bottom of his boat.

Photo by EFRedish, used with permission

This is clearly silly. If he paints the "X" on his boat, it moves with him and it won't help him find the place on the next day. To be able to find it again he needs a marker that is a fixed reference that he can use as a starting point to find the places he wants to find. He needs that starting point, a direction to go in, and a distance to go along that direction. These are what we need to set up a specification of position. We call the way we do it a spatial coordinate system.
A spatial coordinate system
A spatial coordinate system is a very particular kind of graph; it is one in which the points on the graph are meant to correspond to the points in real space  like a map. Most of the graphs we will draw in this class won't be like that but will be more abstract and need interpretation. In a spatial coordinate system, a curve might represent a path an object follows. Since an object can go anywhere, the curve can go back and forth, cross itself, and do lots of other things that graphs in a math class don't usually do. (In math the term coordinate system by itself is often used to represent the axes on any kind of graph and we will also do that.)
It's important to remember that a spatial coordinate system is meant to represent position in physical space.
In general to specify a position, since we live in three dimensional (3D) space, we will need 3 numbers. But for most of the examples we'll deal with in this class, we'll restrict our motions to one or two dimensions (1D or 2D) so we can use a plane.
Even in 2D there are three independent steps to creating a coordinate system tied to a physical space.
 Choose a reference point (origin).
 Choose two axes (called here x and y and taken to be perpendicular to each other)
 Choose a length scale for measuring distances (here taken to be the same in both directions).


Conventions for spatial coordinate systems
There are a number of conventions that we will apply in this class for creating spatial coordinate systems.
 The two axes cross at the origin.
 Sometimes in nonspatial coordinate systems the origin is not shown. This is called a "suppressed zero" and might be used to magnify the variation in a curve. It is often done for the purpose of misleading the viewer into thinking an effect is more important than it really is.
 The positive direction of the axis is indicated with an arrowhead.
 In elementary school and sometimes in high school, axes are drawn with arrows on both ends of the line to indicate "this continues on". Sometimes even the curves have these arrows added to them. Adding arrows on BOTH ends is NOT a professional practice and is NEVER found in scientific journals. If you have acquired this bad habit, divest yourself of it NOW. In professional publications even arrows on one end of an axis are rarely used, but we will use them to get used to thinking about matching a sign to a direction.
 The axes are labeled including specifying the unit in which the axis is measured.
 Because we are mapping something physical, as always, units are extremely important.
These conventions will turn out to be really important since we will be making many different kinds of graphs and things can get very confusing when they are not followed.
Followons
Joe Redish 9/5/11
Wolfgang Losert 9/2/12
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