Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mathematical problem is classical question

  1. Feb 14, 2009 #1
    From the formula 2as = v2 - u2

    I made v the subject...and it becomes -

    (2as+u2)1/2 = v


    Problem is real world value of 2as is negative...as a result it makes a complex answer...but actually its not; instead the value of v should also come negative.

    Here a, u are negative...and a force applies on the body in the same direction as u, which is negative, as a result a negative a cause of that very force.

    So the v should also be real and negative...but its coming as complex :confused:

    Say...can I take the a and u to be positive initially and add the negative sign to the result...sounds ok to me.
     
  2. jcsd
  3. Feb 14, 2009 #2
    [tex]v=\pm \sqrt{2as+u^2}[/tex]
     
  4. Feb 14, 2009 #3

    atyy

    User Avatar
    Science Advisor

    How about the sign of s?
     
  5. Feb 15, 2009 #4
    Yeah its too negative.

    But that way the value that comes by is positive, but it should be negative.
     
    Last edited: Feb 15, 2009
  6. Feb 15, 2009 #5
    See Phrak's response
     
  7. Feb 15, 2009 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Show this explicitly in a specific problem and we will show you where you forgot another sign somewhere. Both Phrak and atyy have given you sufficient hints.

    Zz.
     
  8. Feb 15, 2009 #7

    Doc Al

    User Avatar

    Staff: Mentor

    When 2as is negative that means the body is slowing down. But a negative value of 2as cannot have a magnitude greater than u²--once the body slows to zero it must reverse direction.
     
  9. Feb 16, 2009 #8
    It is accelerating, but all the coordinates are negative, I mean...the u, a and s are all negative.
     
  10. Feb 16, 2009 #9
    Ok then...both negative and positive results will be valid?...I mean...that's true when a value is squared.
     
  11. Feb 16, 2009 #10

    Doc Al

    User Avatar

    Staff: Mentor

    So then what is the issue? 2as is positive, thus v² = u² + 2as is positive. No complex answers required.

    (Realize that this equation only gives you the magnitude of v; the sign is up to you.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Mathematical problem is classical question
  1. Classic problem (Replies: 16)

Loading...