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Homework Help: Determine the angular momentum in polar coordinates

  1. Mar 4, 2017 #1
    Hi there,

    I've been trying to solve the following problem, which I found looks pretty basic, but actually got me really confused about the definition of angular momentum.

    The trajectory of a point mass m is described by the following equations, in spherical coordinates:
    [itex] r(t) = r_0 + v_0t [/itex]
    [itex]\phi(t) = \omega_0t [/itex].
    Determine the angular momentum of m (in spherical coordinates).

    The attempt at a solution
    Angular momentum is defined as [itex] \vec{l} = m\cdot\vec{x(t)}\times\vec{v(t)} [/itex].
    Here, I would say, [itex] \vec{x(t)} = r(t)\hat{r} + \phi(t)\hat{\phi} [/itex].
    I would take the cross product of this vector with its derivative (the speed) multiplied by mass to get the angular momentum.

    But apparently this is wrong. In the given solutions, [itex] \vec{x(t)} = r(t)\hat{r} [/itex]. The angular term is absent.

    Could someone explain why the angular term is set to 0 in the cross product of angular momentum?
    Thank you.
  2. jcsd
  3. Mar 4, 2017 #2


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    Does that make sense dimensionally?
  4. Mar 4, 2017 #3
    Yes it makes sense: in polar coordinates [itex] \hat{\phi} [\itex] is a unit vector perpendicular to [itex]\hat{r}[\itex]. Here is a picture of it dimensionally:
  5. Mar 4, 2017 #4


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    But it is dimensionally wrong. x and r have dimension of length, φ does not.
    ##\vec r## means the same as ##\vec x##. They are both the vector representing the point.

    Or think about it this way: if you start at the origin and go distance r in the direction of the vector r, won't you be at the point?
    Maybe you are getting confused with ##\dot{\vec x}=\dot{\vec r}=\dot r\hat r+r\dot \phi\hat\phi##.
    Last edited: Mar 4, 2017
  6. Mar 4, 2017 #5


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    No, that's not correct. If you have a point [itex]P[/itex] in 2-D space, the associated vector [itex]\vec{r}[/itex] is the vector from the origin to the point [itex]P[/itex]. The coordinate [itex]r[/itex] is just the magnitude of [itex]\vec{r}[/itex], and [itex]\hat{r}[/itex] is the unit vector [itex]\frac{1}{r} \vec{r}[/itex]. So [itex]\hat{\phi}[/itex] doesn't appear at all in the position vector. It does appear in the velocity vector, though:

    [itex]\frac{d \vec{r}}{dt} = \frac{d}{dt} r \hat{r} = \frac{dr}{dt} \hat{r} + r \frac{d \hat{r}}{dt}[/itex]

    [itex]\phi[/itex] comes into play because [itex]\frac{d\hat{r}}{dt} = \frac{d\phi}{dt} \hat{\phi}[/itex]


    [itex]\frac{d \vec{r}}{dt} = \frac{dr}{dt} \hat{r} + r \frac{d \phi}{dt} \hat{\phi}[/itex]

    I'm not sure what's the best way to demonstrate these facts, but you can show that:

    [itex]\frac{d \hat{\phi}}{dt} = - \frac{d\phi}{dt} \hat{r}[/itex]
    [itex]\frac{d \hat{r}}{dt} = \frac{d\phi}{dt} \hat{\phi}[/itex]
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