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Find final speed using mechanical energy formula

  1. Nov 14, 2016 #1
    1. The problem statement, all variables and given/known data
    A skateboarder at a skate park rides along the path shown in (Figure 1) .If the speed of the skateboarder at point A is v=1.3m/s, what is her speed at point B? Assume that friction is negligible.
    fig_8-31.png

    2. Relevant equations
    ui+ki=uf+kf
    u=mgh
    k=1/2mv2

    3. The attempt at a solution
    m(9.8x2.7)+1/2m(1.3)2=m(9.8x1)+1/2mvf2
    m(26.46+.845)=m(9.8+.5) mvf2
    vf=√((26.46+.845)/(9.8+.5))
    answer=1.6 m/s
    The answer is wrong. Can somebody please tell me what I am doing wrong? Thanks!
     
    Last edited by a moderator: Nov 14, 2016
  2. jcsd
  3. Nov 14, 2016 #2
    Looks like you changed 0.845 to 8.45 from one step to the next. That could have caused a problem but I didn't verify that was it.
     
  4. Nov 14, 2016 #3
    I wrote it down wrong here but I did the work using.845 and still got 1.6m/s. I don't know what I am doing wrong :(
     
  5. Nov 14, 2016 #4
    I'll look at it again.
     
  6. Nov 14, 2016 #5
    Yeah, you have a math problem in going from the first line to the second line. You should re-work that again carefully.
     
  7. Nov 14, 2016 #6
    I will do that right now.
     
  8. Nov 14, 2016 #7
    I"m not that great at math. Can you give me a hint of what I did wrong?
     
  9. Nov 14, 2016 #8
    @Angela_vaal you wrote:
    m(9.8x2.7)+1/2m(1.3)2=m(9.8x1)+1/2mvf2
    It's okay that you didn't, but at this point you could have (probably should have) cancelled out all of the m's, because each term contains an 'm'.

    Then you wrote:
    m(26.46+.845)=m(9.8+.5) mvf2
    I'm not exactly sure what you did to get to this equation. It looks like you're trying to factor mvf2 out of both terms on the right-hand side of the equation, but the first term does not contain a vf2. So that was not legal.

    I think you ought to factor out all of the 'm's in the first equation, multiply out the numbers, and that will [hopefully] make it clearer to see how to proceed.
     
  10. Nov 15, 2016 #9

    ehild

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    Gold Member

    The RHS should be 9.8m+0.5 mvf2
    Factor out m.
     
    Last edited: Nov 15, 2016
  11. Nov 15, 2016 #10
    Yeah I think it is just a math error as the general method of solving looks fine
     
  12. Nov 16, 2016 #11
    I figured it out thank you!
     
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