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Crossing paths

  1. Aug 28, 2005 #1
    Runner A is initially 4.54 mi West of a flagpole and is running with a constant velocity of 7.06 mi/h due East. Runner B is initially 4.34 mi East of the flagpole and is running with a constant velocity of 6.82 mi/h due West. Consider East to be in the positive direction. What is the displacement of runner B from the flagpole when their paths cross? Answer in units of mi.

    Thanks.
     
  2. jcsd
  3. Aug 28, 2005 #2

    Tom Mattson

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    Please show how you started, and where you got stuck.
     
  4. Aug 28, 2005 #3
    I figured out that it will take A .643hr to reach the flagpole but B .636hr so I know that when the two cross, it's going to be on the left side of the pole, with negative displacement. At .636hrs, A will be 0.05mi away from the pole so they should cross soon. That's where I'm stuck; I think I'm making this too confusing.
     
  5. Aug 28, 2005 #4

    Tom Mattson

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    You should try to write down the displacement of each runner as a function of time, like this:

    [tex]x(t)=vt+x_0[/tex],

    where [itex]x(t)[/itex] is the displacement, [itex]v[/itex] is the velocity, [itex]t[/itex] is the time, and [itex]x_0[/itex] is the initial displacement. To find out when the runners meet, simply equate the two functions and solve for time.
     
  6. Aug 29, 2005 #5

    SGT

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    What is the initial distance between the two runners?
    What is their relative velocity?
    Dividing the distance by the velocity you find the time they take to cross each other.
     
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