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Relative velocity

  1. Sep 28, 2009 #1
    1. The problem statement, all variables and given/known dat

    So a child is running with the conveyer belt. Relative to the conveyer belt he is running 2.5 m/s.

    2. Relevant equations

    To find the relative velocity of the child to earth, I'm guesing is:

    Velocity of child relative to earth= (v of belt to earth)+(v of child to belt)

    If V child-belt = 2.5, that means he is running 2.5 m/s while on the belt right?

    So if the child is running against the belt at the same speed, is the equation the same? Is the V child-belt still 2.5 but in the opposite direction?

    If i plug in random numbers in the equation for running against the belt, it seems to work still. Say belt is moving 1m/s --> -1.5 m/s= 1 m/s - 2.5 m/s, seems right, he runs -1.5 relative to stationary observers.

    But I would assume, since the belt is coming towards the child, the relative velocity of the child to the belt would be greater than 2.5 m/s? and would equal |Velocity of conveyer belt to earth| + |Velocity of child running (not sure what reference frame now)|?
    Last edited: Sep 28, 2009
  2. jcsd
  3. Sep 28, 2009 #2
    Could you please post some data about the conveyor belt?

    Just remember the definition:


    Which, for your case would translate into:

    [tex]v[/tex]child relative to conveyor [tex]=v[/tex]child relative to earth[tex]-v[/tex]conveyor relative to earth
  4. Oct 1, 2009 #3
    The other information was the length of the conveyor and i was suppose to find the acceleration. But I was just confused about the relative velocity.

    It seems like as long as I use that equation, it works out. Though I still don't quite understand. If you two people running towards each other should it not be their velocities added for their relative velocity to one another?
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