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Homework Help: Couple of relative velocity problems.

  1. Sep 25, 2009 #1

    A swimmer heads directly across a river, swimming at 1.7 m/s relative to the water. She arrives at a point 44 m downstream from the point directly across the river, which is 76 m wide.

    (a) What is the speed of the river current?
    1 m/s

    (b) What is the swimmer's speed relative to the shore?
    2 m/s

    I have my picture all drawn out but I don't understand the material at all.

    2. This one is easy but my answer keeps coming out wrong ?

    The range of a cannonball fired horizontally from a cliff is equal to the height of the cliff. What is the direction of the velocity vector of the projectile as it strikes the ground? (Ignore any effects due to air resistance.)

    3. Last one I couldn't do.

    While walking between gates at an airport, you notice a child running along a moving walkway. Estimating that the child runs at a constant speed of 2.7 m/s relative to the surface of the walkway, you decide to try to determine the speed of the walkway itself. You watch the child run on the entire 25-m walkway in one direction, immediately turn around, and run back to his starting point. The entire trip takes a total elapsed time of 33 s. Given this information, what is the speed of the moving walkway relative to the airport terminal?

    So brain dead when it comes to physics, sigh.
  2. jcsd
  3. Sep 25, 2009 #2


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    Science Advisor
    Homework Helper

    Hi Raheelp ! :smile:

    1: Relative velocities are vectors, so they obey the laws of vector addition.

    I assume you've drawn a vector triangle … what are the sides and angles of it?​

    2: Show us your full calculations, and then we can see what went wrong, and we'll know how to help. :smile:

    (3, we'll leave for the present)
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