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Homework Help: Conservation of energy question please help!

  1. Jun 16, 2010 #1
    1. The problem statement, all variables and given/known data

    An 8.70kg block slides with an initial speed of 1.66m/s up a ramp inclined at an angle of 27.4 degrees to the horizontal. The coefficient of kinetic friction between the block and the ramp is 0.3. Use energy conservation to find the distance the block slides before coming to rest.

    2. Relevant equations

    I think Wf=/F//delta s/cos(F,delta s) is relevant, but I'm unsure how to apply it.

    3. The attempt at a solution

    My answers don't really make sense (I have them written down on paper, but are too complicated to write online). Im not really sure how to make use of the coefficient of kinetic friction. Any help would be greatly appreciated.
  2. jcsd
  3. Jun 16, 2010 #2
    W = f dot d

    Where is the confusion? Be more explicit (as in show work or ask less broad questions)
  4. Jun 16, 2010 #3
    I apologize for the vagueness of my questions, this was my first time using this forum. So, I essentially know how to do the conservation of energy part of the question (although it would be appreciated if someone could go through it to confirm that my work so far is correct). But I'm not sure what role the coefficient of kinetic friction plays in the answer. So if someone could elaborate on that, I would really appreciate it.
  5. Jun 16, 2010 #4
    Hi. Let's not let me show all work. Instead, I will walk through the problem.

    First look at your triangle. You need to have the right labels for known values (given forces, component forces, and angles). It does not matter whether we will use all of them of not (one or two may not applicable, who knows).

    This assumes that you other than the given forces should have 1) horizontal component due to gravity 2) vertical component due to gravity

    Let us remember there are two different energy of conversations (actually they are the same equation). The reason I said two are not to confuse you at this point, and not to draw the attention.

    If there is no friction involves, we say the system should have energy conserved by
    Kf + Uf = Ki + Ui
    K is the the kinetic and U is the potential energy

    If friction presents, we no longer have a conserved equation, that is
    changes of K + changes of U = changes of mechanical energy = - Ff * d

    Which in general, is the same form as the first equation. When there is no friction or any nonconservative force presents, we can move some terms over the other side.

    Look at your picture again. What do you need for your equations?
    Use those components.

    Hint and tips: don't worry about the answer yet. I want you to develop the concept before actually finding the right answer (if you fail to do it this time).

    Ps: I suggested you make a triangle and attach it (screenshot it, or whatever method...) here if you can't find the right answer after your second attempt
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