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Final Speed Question

  1. Feb 27, 2015 #1
    1. The problem statement, all variables and given/known data
    An object with mass 161kg moved in outer space. When it was at a location [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png7,-17,-9[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngm [Broken] its speed was 18m[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmmi10/alpha/144/char3D.pngs. [Broken] A single constant force [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png143,308,-396[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngN [Broken] acted on the object while it moved to a location [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png14,-25,-15[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngm. [Broken] What is the speed of the object at this final location?



    2. Relevant equations
    W=Fd
    F=ma
    KE=1/2mv^2
    v^2=u^2+2as

    3. The attempt at a solution

    1) Used F=ma
    <143,308,-396> = 161a
    a=<.888,1.913,-2.46>m/s

    2) Plugged numbers into v^2=u^2+2as to get final velocity, I did it 3 times because of vectors so I got Vf= <18.342,17.192,18.802>

    3) I took <18.342,17.192,18.802> and did sqrt(18.342^2+17.192^2+18.802^2) and got 31.358m/s
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Feb 27, 2015 #2

    TSny

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    Hello, Westin.

    For the x-component, you would have ##v_x^2 = u_{x}^2 + 2a_x\Delta x##. But, you don't know ##u_x##.

    I think work and energy concepts are going to be a better approach here.
     
  4. Feb 27, 2015 #3
    The section we are covering is currently energy so I think you're right.

    Second attempt:
    1) W=Fd
    W= (<143,308,-396> )(<7,-8,-6>)
    W= <1001,-2464,2376>

    2) Kfinal = Kintial + W

    Kinital = (1/2)mv^2 = (1/2)(161)(18)^2 = 26082J

    Kfinal = 26082J + <1001,-2464,2376>J

    Kfinal = <27083, 23618, 28458> J

    3) Kfinal / mass = Final velocity
    <27083, 23618, 28458> / 161 = <168.217, 146.696, 176.758>m/s

    4) sqrt(168.217^2+146.696^2+176.758^2) = 284.71m/s

    Incorrect once again :/




     
  5. Feb 27, 2015 #4

    TSny

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    Work is a scalar quantity, not a vector quantity.
     
  6. Feb 27, 2015 #5
    does everything else look good besides that?
     
  7. Feb 27, 2015 #6

    TSny

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    You have the correct overall approach.
    But note that KE is also a scalar quantity!
     
  8. Feb 27, 2015 #7

    TSny

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    Check this.
     
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