Results 1 to 3 of 3

Math Help - simple vectors

  1. #1
    Member
    Joined
    May 2009
    Posts
    91

    Red face simple vectors

    Hi folks,

    A ship travels 48 km in a certain time period against a current with an average speed of 6 km/h and 72 km with the stream. The question is what is the speed of the ship when there is no current. I'm stuck!

    This should be easy: distance s = v (velocity) x t (time)

    48 = (v - 6) t
    72 = (v + 6) t

    so

    48 = vt - 6t
    72 = vt + 6t

    subtracting

    120 = 2vt, so the ship travels 60 km is still water. But I can't figure out how to calculate the velocity!
    It seems that going with the stream, the ship makes an extra 12 km, and at 6 km/h this would take 2 hours.
    so in still water

    60 = vt
    60 = v.2

    so v = 30 km/h

    Sorry, figured the problem out myself. I would delete this but apart from blanking out the entire post, I don't kinow how! If anyone knows how to delete a post, I wopuld be pleased to know.
    Last edited by s_ingram; October 25th 2009 at 05:22 AM. Reason: problem already solved
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,807
    Thanks
    116
    Quote Originally Posted by s_ingram View Post
    ...

    A ship travels 48 km in a certain time period against a current with an average speed of 6 km/h and 72 km with the stream. The question is what is the speed of the ship when there is no current. I'm stuck!

    This should be easy: distance s = v (velocity) x t (time)

    48 = (v - 6) t
    72 = (v + 6) t

    ...
    Even though you got the correct answer I'll do this question nevertheless:

    48 = (v - 6) t~\implies~t=\dfrac{48}{v-6}
    Thus:
    72=(v+6) \cdot \dfrac{48}{v-6}~\implies~72(v-6)=48(v+6)~\implies~24v=720

    Solve for v.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    May 2009
    Posts
    91
    Hi Earboth. Yes yours is a neater solution.
    By the way, is it possible for me to delete my own post?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. simple explanation about translation vectors
    Posted in the Calculus Forum
    Replies: 5
    Last Post: January 15th 2012, 10:41 PM
  2. Simple 3D Vectors question!
    Posted in the Pre-Calculus Forum
    Replies: 1
    Last Post: March 4th 2010, 12:13 PM
  3. Vectors and simple probability
    Posted in the Statistics Forum
    Replies: 2
    Last Post: November 3rd 2009, 07:48 AM
  4. a simple problem about vectors
    Posted in the Calculus Forum
    Replies: 0
    Last Post: October 11th 2008, 10:28 PM
  5. Simple vectors
    Posted in the Pre-Calculus Forum
    Replies: 7
    Last Post: June 19th 2008, 11:11 AM

Search Tags


/mathhelpforum @mathhelpforum