1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Relative circular motion

  1. Feb 25, 2006 #1
    • Two cyclists( A, B) traveling with the same constant speed, v,
    • in a circular track.
    • They start at the same point on the circle.
    • cyclist B travel through the diameter of the circle, assume the x-axis on a xy
    • Cyclist A travel on the circumference of the circle

    • Find speed of A with respect to B.

    Given: the constant speed, radius, angle between A and the x-axis through the centre, angle between A and B.

    am I suppose to do something with the acceleration (normal/centripedal) and the two angles given.

    thank you
  2. jcsd
  3. Feb 26, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    It has to do with the velocity of each, and the velocity of A with respect to B is dependent on the angles between the velocity vectors.

    When A and B start, A is moving away from B, and only starts moving toward B after A passes the quarter arc.

    At time t = v/D = v/2R, B moves outside A's circular trajectory.

    [itex]\vec_B[/itex] is always v[itex]\,\hat{x}[/itex], whereas

    [itex]\vec_A[/itex] is always v[tex]\,\hat{\theta}[/tex] where [tex]\hat{\theta}[/tex] is the unit vector in the azimuthal direction (tangent to circumference of circle). As xB gets very large, the angle between A and B gets very small.

    The 'speed' would be the magnitude of the velocity vector given by [itex]\vec_A[/itex] - [itex]\vec_B[/itex]
  4. Feb 27, 2006 #3
    Thanks for the reply. It really does look a lot simpler now.

    But when you wrote t= V/D, did you mean t = D/V?
  5. Mar 1, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, t = D/V. My mistake.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook