# Polar coordinates: e_r and e_theta

#### sara_87

1. Homework Statement [/b]

Let e_r=(cos$$\theta$$,sin$$\theta$$) and e_theta=(-sin$$\theta$$,cos$$\theta$$).
Let P(r,$$\theta$$) be a point with e_r and e_theta at that point.
What can you say about the three quantities (e_r, e_theta and the point P) as r and $$\theta$$ vary?

2. Homework Equations

r: distance from origin
$$\theta$$: angle

3. The Attempt at a Solution

As r moves around, the e_r and e_theta change place. As r increases or decreases e_r and e_theta don't change place but as theta changes, they do change place since e_theta is always orthogonal to point P.

I feel like I'm not putting enough information. Is there something i didn't mention?
Thank you.

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#### djeitnstine

Gold Member
Yes there is some information missing. You have to note that as r changes, $$\theta$$ changes with the rate that r changes, because $$e_{\theta}$$ is the derivative of $$e_{r}$$

#### sara_87

What do you mean, I dont understand why.

#### djeitnstine

Gold Member
Ok, starting with a (position) vector function, (I will use the cartesian coordinate system to make things appear obvious) lets say $$v=t^{2}i+2tj$$ The derivative is $$v'=2ti+2j$$. Then how does $$v$$ vary with $$v'$$? Since $$v'$$ is the derivative of $$v$$, then $$v'$$ must vary with $$v$$'s rate of change. Looking at your position functions, $$-\sin{(\theta)}$$ is the derivative of $$\cos{(\theta)}$$. Likewise for $$sin{(\theta)}$$ and $$cos{(\theta)}$$

Oh right, i see.
Thank you

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