1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Swimming Across River Question

  1. Sep 23, 2013 #1
    1. The problem statement, all variables and given/known data
    A man can swim through still water 4.2m/s. Suppose this man swims in a river with current 1.3 m/s. Find his average speed over a trip:
    a) across the river and back
    b) a certain distance downstream and back



    3. The attempt at a solution

    a)
    Here is a diagram I made:
    106kmlv.png
    Left side of picture is there, right side is back

    So should I solve for x, then take x, multiply by two, then divide by two. Thats the avg speed? because the question doesnt give any distance or time, so this is all I could think of.

    b) is this a vector addition question? so his avg speed would be 2.9m/s?

    Thanks
     
  2. jcsd
  3. Sep 23, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    a. Avg speed is x, right.
    b. Careful with this one. Remember, avg speed = total distance/total time. Does not equal 2.9.
     
  4. Sep 23, 2013 #3
    So can I choose any distance? Since I am not given one.
     
  5. Sep 23, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Right.
     
  6. Sep 23, 2013 #5
    Ok. I chose 16m as my distance downstream.

    After computing it I get 4.1m/s as the avg speed for the trip
     
  7. Sep 23, 2013 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Not what i got. Show your work?
     
  8. Sep 23, 2013 #7
    ok first i add the vectors. On the downstream his velocity increases to 7.1m/s while the upstream is 2.9 m/s.

    Using these two found values i calculate time for the 16m upstream and 16m downstream. using t = d/v

    For upstream I get 5.52s and downstream i get 2.25s.

    Now I do avg spd. total distance/total time. I add up the distances i get 32m, i add up the times i get 7.77s. 32/7.77s I get 4.12m/s

    is this not correct?
     
  9. Sep 23, 2013 #8

    rude man

    User Avatar
    Homework Helper
    Gold Member

     
  10. Sep 23, 2013 #9
     
  11. Sep 23, 2013 #10

    rude man

    User Avatar
    Homework Helper
    Gold Member

     
  12. Sep 23, 2013 #11
    As I read the problem, "across the river and back" to me means that he ends up at his starting point. Your diagram indicates he would end up 2.6 downstream. In order for him to arrive back at his starting point you would have to interchange the 4.2 m/s and x on your diagram.
     
  13. Sep 23, 2013 #12
     
  14. Sep 23, 2013 #13
    yes you are right. I now get 4.2m/s as the penguins average speed for part a
     
  15. Sep 24, 2013 #14
    Is this number correct?
     
  16. Sep 24, 2013 #15

    rude man

    User Avatar
    Homework Helper
    Gold Member

     
  17. Sep 24, 2013 #16
    How did you get 3.8 m/s? Can you show your work? I get a value slightly higher.

    What did you get for part b?
     
  18. Sep 24, 2013 #17
    I got 3.8 for part b. For part a I got 4.2m/s.
     
  19. Sep 24, 2013 #18

    rude man

    User Avatar
    Homework Helper
    Gold Member

    That was part b. Same as what you got.

    I never answered part a but 4.2 looks wrong. 4.2 is his speed in calm water. I think the answer is the same as what you would have gotten in your original post: x^2 = 4.2^2 + 1.3^2.
     
  20. Sep 24, 2013 #19
    Are you sure? I think skeptic may have a point about having to return to the same starting point.
     
  21. Sep 24, 2013 #20

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Yes, but I think the answer's the same. He would start upstream by a certain distance and wind up at the starting point, and the velocity vectors would be the same as if he wound up downstream by the same distance.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Swimming Across River Question
Loading...