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Homework Help: 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.

    2. Relevant equations

    3. The attempt at a solution
    m(26.46+.845)=m(9.8+.5) mvf2
    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:
    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


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    Homework Helper

    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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