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Mom of Special Needs High School Student NEEDS HELP

  1. Sep 23, 2007 #1
    An example of the work he's trying to do is:


    1. A stream's current has a velocity of 10 km/hr. You and a friend are trying to paddle upstream at 6 kn/hr. What is the resulting velocity.



    2. My problem is that I cannot find the formula or equation that he needs to figure out the problem.



    3. There has been no attempt at a solution.


    My son is 14, Autistic, and a freshman in high school. It's been seventeen years since I took physics and I just don't remember the information well enough to help him. He's pretty savant-ish when it comes to math. If he has the formula, he can do the work.

    Please help us.

    Christina
     
  2. jcsd
  3. Sep 23, 2007 #2
    Draw a picture, it always helps when doing physics problems.

    Basically you have a stream traveling down at 10km/hr(we well call it -10 km/hr because it is going down) and you are going up at 6 km/hr, the resulting velocity would be the sum of the 2. So -10 km/hr + 6 km/hr = -4 km/hr or 4 km/hr [with the stream].
     
  4. Sep 23, 2007 #3
    so, basically it's this formula:

    velocity relative to x = velocity of a relative to x + velocity of b relative to x

    except when you have velocities in opposite directions one of the values is negative. How do you decide which one is negative?
     
  5. Sep 23, 2007 #4
    You just pick really, unless it tells you the direction the stream is traveling(N, E, S, W, left or right). If it just says there is a stream you just pick. Or you can just subtract the 2 positive values for the same result.
     
  6. Sep 23, 2007 #5

    Astronuc

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    Staff: Mentor

    Welcome to PhysicsForums, Christina

    Velocities are vector quantities (magnitude and direction).

    Is 6 kn/hr supposed to be 6 km/hr? or is kn = knots. There's a difference.

    Upstream is opposite downstream, so one could take downstream as negative and adding the velocities, -10 km/hr + 6 km/hr = - 4 km/hr, so the net vecocity of the craft is 4 km/hr downstream.

    The significance of the + or - signs has to do with orientation.

    This site might be of use - http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

    Specifically for the current problem:

    Vectors - http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html

    Boat in current - http://hyperphysics.phy-astr.gsu.edu/hbase/boatc.html

    Velocity - http://hyperphysics.phy-astr.gsu.edu/hbase/vel2.html

    Relative velocity - http://hyperphysics.phy-astr.gsu.edu/hbase/relmot.html
     
  7. Sep 23, 2007 #6
    Thank you thank you thank you! both of you have been very helpful, but these links are priceless ... just what I needed to be able to help him with his work. I can't tell you how grateful I am. Thanks again!!!
     
  8. Sep 23, 2007 #7

    Astronuc

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    Staff: Mentor

    You just did, and you are welcome. Good luck with your son.
     
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