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Homework Help: Graph of Position Seems to be Wrong

  1. Sep 16, 2018 at 11:06 AM #1
    1. The problem statement, all variables and given/known data

    By relative velocity, we mean velocity with respect to a specified coordinate system. (The term velocity, alone, is understood to be relative to the observer’s coordinate system.)

    (a) A point is observed to have velocity Va relative to coordinate system A.What is its velocity relative to coordinate system B, which is displaced from system A by distance R? (R can change in time.)

    (b) Particles a and b move in opposite directions around a circle with angular speed ω, as shown. At t = 0 they are both at the point r = lˆj, where l is the radius of the circle. Find the velocity of a relative to b.

    (c) Sketch the motion of the particle.

    2. Relevant equations


    3. The attempt at a solution

    I was able to solve part A and found Va=Vb + dR/dt. I'm pretty confident in this answer.

    I was also able to solve part B. I found dR/dt to be 2wlcos(wt). I then plugged this into the answer from part A and got Va=Vb + 2wlcos(wt). I'm pretty confident in this answer as well.

    My issue is with part C. To graph the position, I integrated the answer from part B and got Rx=Vbt+2lsin(wt). I believe I did this right, but if you graph it the sin function does not go linearly across the graph as it normally does, but instead gradually increases due to the Vbt term in front of it. This does not seem to make sense in the real world because the particles are going around a circle so they should not be gradually getting farther apart over time? I'm pretty stumped on this. Any input about this would be appreciated.

    Thanks for all the help!
  2. jcsd
  3. Sep 16, 2018 at 12:18 PM #2

    Stephen Tashi

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    If we are assuming particle B's coordinate system places (at all times) particle B at the origin of that system, then particle B's velocity in that system is zero.
  4. Sep 16, 2018 at 12:30 PM #3
    So since particle B's position would always be (0,0), it could just be left out of all the equations then? Including in part A?
  5. Sep 16, 2018 at 12:39 PM #4

    Stephen Tashi

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    No. In part a), I interpret the phrase "coordinate system B, which is displaced from system A by distance R" to mean the origin of coordinate system B is displaced from the origin of coordinate system A by displacment vector R. That does not imply that when we do computations of a particle's velocity that we always assume the particle is at the origin of one of the coordinate systems.

    In the special case where a problem asks about something "relative to particle A", it usually means we do use a coordinate system where the particle is at the origin of "its" coordinate system.
  6. Sep 16, 2018 at 12:42 PM #5
    Thanks that helps a lot. Also, in part B it is talking about being relative to particle B so the Vb term could be left out since it would imply that it is 0?
  7. Sep 16, 2018 at 12:45 PM #6

    Stephen Tashi

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    The way I'd phrase it is that the Vb is zero so the Vb term has no effect. "Left out" implies something about notation, which may or may not get the approval of a grader.
  8. Sep 16, 2018 at 12:47 PM #7
    Thanks for all the help!
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