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Reference Frames: Up and Down a River

  1. Oct 11, 2014 #1
    1. The problem statement, all variables and given/known data

    A water spider maintains an average position on the surface of a stream by darting upstream (against the current), then drifting downstream (with the current) to its original position. The current in the stream is 0.408 m/s relative to the shore, and the water spider darts upstream 0.558 m (relative to a spot on shore) in 0.844 s during the first part of its motion. Use upstream as the positive direction.

    a) Find the velocity of the water spider relative to the water during its dash upstream.
    Answer in units of m/s

    b) What is its velocity (relative to the water) during its drift downstream?
    Answer in units of m/s

    c) How far upstream relative to the water does the water spider move during one cycle of this upstream and downstream motion?
    Answer in units of m

    d) What is the average velocity of the water spider relative to the water for one complete cycle?
    Answer in units of m/s

    2. Relevant equations

    -None-

    3. The attempt at a solution

    I believe I am doing this right, but when I submit my answers online, the website says they are all (a-d) wrong. Am I doing something wrong?

    V(of spider relative to ground)=.558m/.844s = .6611

    a) V(of spider relative to water)= V(of spider relative to ground) -V(of ground relative to water)
    VSW=VSG-VGW
    VSW=.6611-.408=.2531 m/s

    b) VSW=VSG-(-VGW)
    VSW=.6611+.408=1.0691 m/s

    c)DSW=(VSW)t
    DSW=(.2532)(.408)=.2336 m

    d) AverageSW= (.2531+1.0691)/2 = .6611 m/s
     
  2. jcsd
  3. Oct 12, 2014 #2

    ehild

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    Homework Helper
    Gold Member

    The spider moves upstream while the water flows downstream. If the velocity f the spider is positive, that of the water is negative.
    You have never tried to swim upstream? you have to swim hard just to stay at the same place with respect to the ground...

    Read the question carefully:
    What does drift mean ?
    It is explained in the first sentence of the problem:
     
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