Problems for
Intermediate Methods in Theoretical Physics

Edward F. Redish

Changing coordinates: Circular motion

A recurring theme in mathematical physics is that while we consider that a physical system exists independent of how we choose to represent it mathematically, that choosing one particular mathematical representation vs another might make things look a lot simpler. Since these representations are different ways of looking at the same thing, you have to be able to go from one to the other by mathematical transformations (in principle -- when you can't, there's often new physics hiding buried in the math and the math is trying to tell you that). Let's try this in a straightforward example: circular motion.


(a) Consider a vector describing an object's position in a two space:


We want to consider the object moving around the origin in a circle, so it's distance from the origin

is a constant, even though x(t) and y(t) are functions of time. Let's create a new polar coordinate basis, with the first unit vector along the direction towards the point and the second perpendicular to it as shown in the figure at the right. Express the unit vectors (note that these change as the position of the object changes) in terms of the fixed unit vectors.

(b) Invert the expressions you derived in (a), now expressing the unit vectors in terms of the unit vectors . Put these into the original definition of the position vector so you can express it in terms of the new basis. Does the result surprise you? Is it reasonable?

(c) Now consider that the particle is moving in the circle determined by the polar equations

r = constant
θ = ω t

where ω is a constant. Construct the vector velocity by differentiating with respect to time. Do it in two ways: first by differentiating the expression for the vector in terms of of the x-y basis and second by differentiating the expression for the vector in terms of the r-θ basis. Show that the answers you get are the same.

This page prepared by

Edward F. Redish
Department of Physics
University of Maryland
College Park, MD 20742
Phone: (301) 405-6120

Last revision 4. November, 2004.