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Homework Help: Galilean transformation problem (Speed)

  1. Apr 18, 2014 #1
    1. The problem statement, all variables and given/known data

    A girl is riding a bicycle along a straight road at constant speed, and passes a friend standing at a bus stop (event #1). At a time of 60 s later the friend catches a bus (event #2)
    If the distance separating the events is 126 m in the frame of the girl on the bicycle, what is the bicycle's speed?

    2. Relevant equations
    u = u' + v

    3. The attempt at a solution
    u = u' + v
    can be written as:
    Δx/t = Δx'/t + v
    v = Δx/t - Δx'/t
    v = 0m/60s - 126m /60s
    v = -126 m/ 60s
    v = -2.1 m/s

    Just wondering if the negative holds any significance? I know we're talking about speed which is scalar but how come the calculation gives a negative?
  2. jcsd
  3. Apr 18, 2014 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    It should be +2.1 m/s. This is because the Δx' represents the displacement of the second event minus the displacement of the first. So v = 0/60 - (-126/60).

    You are using the cyclist's reference frame with the origin at the cyclist to determine u' and the bus stop person's (bsp) reference frame with the bsp at the origin to determine u. There are two events: 1. the origins coincide and 2. the bsp enters the bus.

    In the cyclist's frame these events occur at 0 and -126 m. using the direction of v as the +x direction. So Δx' = -126m. In the bsp's frame, they both occur at the origin.

  4. Apr 19, 2014 #3
    Why wouldn't it be +126 m if she is moving forward?
  5. Apr 19, 2014 #4

    Doc Al

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    Staff: Mentor

    To the cyclist, the ground is moving backwards.
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