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How velocities transform from Cartesian to Polar coordinates

Physics Asked on May 9, 2021

Consider a transformation from Cartesian to polar coordinates $(x,y)rightarrow (r,theta)$,
begin{equation}
begin{gathered}
x=rcostheta,
y=rsintheta.
end{gathered}
end{equation}

Here, we denote $x^{,mu}=(x,y)$ and $bar{x}^{,mu}=(r,theta)$.
Now, The question is the following,

In the $x^{,mu}$ coordinate system, the components of the velocity vector are $(dot{x},dot{y})$. Find out the components in the polar coordinates using vector/tensor transformation rules.

My answer:

From the coordinate transformation we have,
begin{equation}
begin{gathered}
dx=costheta dr-rsintheta dtheta,
dy=sintheta dr+rcostheta dtheta.
end{gathered}
end{equation}

Thus,
begin{equation}
begin{gathered}
frac{partial x}{partial r}=costheta=frac{x}{r};quad frac{partial x}{partial theta}=-rsintheta=-y,
frac{partial y}{partial r}=sintheta=frac{y}{r};quad frac{partial y}{partial theta}=rcostheta=x.
end{gathered}
end{equation}

The transformed components $bar{V}^{,mu}=bar{V}^{,mu}(x^{,alpha})$ reads,
begin{align}
bar{V}^{,mu}=frac{partial, bar{x}^{,mu}}{partial, x^{,beta}}V^{,beta}
end{align}

Now, for $mu=1$,
begin{align}
bar{V}^{,1}&=frac{partial, bar{x}^{,1}}{partial, x^{,beta}}V^{,beta}nonumber
&=frac{partial, bar{x}^{,1}}{partial, x^{,1}}V^{,1}+frac{partial, bar{x}^{,1}}{partial, x^{,2}}V^{,2}nonumber
&=frac{partial r}{partial x}V^{,1}+frac{partial, r}{partial y}V^{,2}nonumber
&=sectheta V^{,1}+csctheta V^{,2}nonumber
&=frac{r}{x} V^{,1}+frac{r}{y} V^{,2}
tag{1}label{eq:comptransone}
end{align}

Now, for $mu=2$,
begin{align}
bar{V}^{,2}&=frac{partial, bar{x}^{,2}}{partial, x^{,beta}}V^{,beta}nonumber
&=frac{partial, bar{x}^{,2}}{partial, x^{,1}}V^{,1}+frac{partial, bar{x}^{,2}}{partial, x^{,2}}V^{,2}nonumber
&=frac{partial theta}{partial x}V^{,1}+frac{partialtheta}{partial y}V^{,2}nonumber
&=-frac{1}{r}csctheta V^{,1}+frac{1}{r}sectheta V^{,2}nonumber
&=-frac{1}{y} V^{,1}+frac{1}{x} V^{,2}
tag{2}label{eq:comptranstwo}
end{align}

begin{equation}
begin{gathered}
dot{x}=costheta dot{r}-rsintheta dot{theta},
dot{y}=sintheta dot{r}+rcostheta dot{theta}.
end{gathered}
end{equation}

Now, we calculate the velocity components in the polar coordinates using equations ($ref{eq:comptransone}$) and ($ref{eq:comptranstwo}$),
begin{align}
v^{,r}&=sectheta dot{x}+cscthetadot{y}nonumber
&=secthetaleft(costheta dot{r}-rsintheta dot{theta}right)+cscthetaleft(sintheta dot{r}+rcostheta dot{theta}right)nonumber
&= dot{r}-rtantheta dot{theta}+dot{r}+rcottheta dot{theta}nonumber
&= 2dot{r}-r(tantheta -cottheta) dot{theta}
end{align}

begin{align}
v^{,theta}&=-frac{1}{r}csctheta dot{x}+frac{1}{r}sectheta dot{y}nonumber
&=-frac{1}{r}csctheta left(costheta dot{r}-rsintheta dot{theta}right)+frac{1}{r}sectheta left(sintheta dot{r}+rcostheta dot{theta}right)nonumber
&=-frac{1}{r}cotthetadot{r}+dot{theta}+frac{1}{r}tanthetadot{r}+dot{theta}nonumber
&=2dot{theta}+frac{dot{r}}{r}(tantheta-cottheta)
end{align}

Present question: Are the above equations which I derived correct? Shouldn’t this be something like $v^r=dot{r}$ and $v^theta=rdot{theta}$? Where am I going wrong? Help please.

One Answer

The issue with the velocity transformation is resolved if you use the matrix inverse of the Jacobian. In your case, note that the inverse transformation you are using involves terms like $$ frac{partial r}{partial x}Biggrvert_theta = sec theta $$ which does not make much sense since $ theta = theta(x,y) $ is also a function of $x$ and $y$. The issue is resolved by taking derivatives from the inverse functions directly $$ r = sqrt{x^2+y^2} implies frac{partial r}{partial x}Biggrvert_y = cos theta $$ which reproduces the inverse Jacobian matrix elements.

You will get $$ bar{V}^mu = (dot r, dot theta) = dot r partial_r + dot theta partial_theta $$ as we expect because we should be able to do the time-derivative in any coordinates, and then noting that $$ partial_theta = r hat theta $$ and $$ partial_r = hat r $$ you recover the usual expression from vector calculus $$ dot r hat r + r dot theta hat theta $$

Correct answer by jesseylin on May 9, 2021

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