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!

Problem with vectors

  1. Feb 8, 2007 #1
    The velocity of a particle moving in the xy plane is given by = (5.6 t - 4.5 t2) + 8.2, with in meters per second and t (> 0) in seconds. (a) What is the acceleration when t = 3.3 s? (b) When (if ever) is the acceleration zero? (c) When (if ever) does the speed equal 10 m/s?

    I got the first 2 but when I went to find c I can't seem to get it.

    I took

    10m/s = sqrt((5.6t - 4.5t^2)^2 + (8.2)^2
    10^2 = (sqrt((5.6t - 4.5t^2)^2 + (8.2)^2)^2
    100 = (5.6t - 4.5t^2)^2 + 67.24)
    32.76 = (5.6t - 4.5t^2)^2
    sqrt(32.76) = sqrt((5.6t - 4.5t^2)^2)
    5.72 = 5.6t - 4.5t^2
    0 = -4.5t^2 + 5.6t - 5.72
    then I used quadratic to find out the two values...
    but I get a negative in the sqrt on the quadratic and I can't do that... any suggestions?
     
  2. jcsd
  3. Feb 8, 2007 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    You implicitly assumed [itex]t<=56/45[/itex]
    when you wrote
    [tex]\sqrt{(5.6t-4.5t^2)^2} = 5.6t-4.5t^2[/tex]
    What happens if [itex]t>56/45[/itex]?
     
  4. Feb 8, 2007 #3
    so how do you figure it out? I'm not sure how you would go about doing it.

    Could you take

    5.6t-4.5t^2
    t(5.6-4.5t)
    t = 5.6/4.5 ?

    if so what happens with the 5.72 on the other side of the equation?
     
  5. Feb 8, 2007 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    My hint wasn't good enough.

    It is not valid to write [itex]\sqrt{(5.6t-4.5t^2)^2} = 5.6t-4.5t^2[/itex] when [itex]t>56/45[/itex] because [itex]5.6t-4.5t^2[/itex] is negative.

    For example, try [itex]t=2[/itex]. The velocity vector at [itex]t=2[/itex] is [itex][ 5.6*2 - 4.5*2^2, 8.2 ] = [ -6.8, 8.2 ] [/itex]. Note that the [itex]x[/itex] component of the velocity is negative. [itex]\sqrt{(-6.8)^2} = 6.8,\text{\ not\ } -6.8[/itex].
     
    Last edited: Feb 8, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Problem with vectors
  1. Vector problem (Replies: 8)

  2. Vector Problem (Replies: 3)

  3. Vector problem (Replies: 8)

  4. Vector problem (Replies: 1)

Loading...