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Homework Help: Clockwise or counterclockwise motion?

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Th path of the particle P is given by the following equation:

    [itex] r = <cos(t),sin(t)> [/itex] defined for all t>0

    Is the particle travelling clockwise or counterclockwise as t increases?

    2. Relevant equations

    3. The attempt at a solution

    I graphed the function, and it is a circle, initially the particle is at the point <1,0> and it moves counterclockwise according to the graph.

    But how could I find whether it's travelling clockwise or counterclockwise in general, for any parametric curve?

    Might it have anything to do with cross products? This is all assuming the particle's motion lies strictly in the xy plane.

  2. jcsd
  3. Nov 14, 2012 #2


    Staff: Mentor

    Look at
    $$ \frac{d \bf{r}}{dt}$$

    It will be pointing in the direction of increasing t.
  4. Nov 14, 2012 #3
    Hey Mark, thanks. I know that v is parallel to the direction of the particle for increasing t. I can calculate it by differentiating each component of r with respect to t.

    However, once I know r and v (as functions of t), how can I use them to decide whether the particle travels clockwise, or how can I determine for what values of t the particle travels clockwise?

    My friend suggested polar coordinates but I don't know if that works. My idea was cross product of r and v, but is that correct?

  5. Nov 14, 2012 #4
    For the example you gave, you just need to know the basic properties of sine and cosine. But in general, if you want to know whether your curve is turning left or right, you need to calculate the acceleration r''(t). If r' cross r'' is positive then that means you are turning left. If r' cross r'' is negative that means it is turning to the right. The cross product comes out to be: x' y'' - y' x''. So you just check whether that is positive (counterclockwise=left turn) or negative (clockwise=right turn)
  6. Nov 14, 2012 #5


    User Avatar
    Science Advisor
    Homework Helper

    If you are dealing with a plane curve, then sure. The sign of the k component of rxv will tell you which direction you are travelling around the origin.
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