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Finding the Transferred Energy for a Block Sliding Down a Ramp

  1. Feb 27, 2014 #1
    1. The problem statement, all variables and given/known data

    A 5.0kg block slides down a ramp, starting with a velocity down the slope of 2.5 m/s. The ramp is 1.5 m high and has an angle of 25 degrees. The force of friction acting upon the block is 20.0 N.

    How much energy is transferred in or out by gravity, normal force, and friction respectively as it slides down the ramp?

    2. Relevant equations

    Ek = 1/2 mv2
    Ep = mgh
    Fg = 9.8m
    Fn = Fgx
    Ff =μFn

    3. The attempt at a solution
    This question is actually a five part question. I have completed parts 1 and 5, which is here in case it may be useful:
    1: How much energy does the block begin with?
    A: Eki= 0.5(5.0kg)(2.52) = 15.6 J
    5. What is the final velocity of the block?
    Ef = Ei + WFf
    (0.5)(5.0kg)vf2 = (5.0kg)(9.8m/s/s)(1.5m) + (0.5)(5.0kg)(2.52m/s) - (20N)(1.5/cos(25)m)
    vf = 2.69 m/s

    I'm not sure what concept I'm supposed to apply to discover "energy transferred", however. I thought that the energy within a closed system remained constant. I do not want the answers; I simply want some sort of push in the right direction.
     
  2. jcsd
  3. Feb 27, 2014 #2

    Dick

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    Homework Helper

    I think the "system" they want you to think about is just the block, and that system is not closed. You know the initial and final kinetic energies of the system. They are asking about the net effect of each of those three forces to the kinetic energy of the block. Since you are going down, gravity can only put energy in. Friction can only take energy out. Can normal force do either? The answers to those are in the way you computed vf. Is that an ok push?
     
    Last edited: Feb 27, 2014
  4. Feb 27, 2014 #3
    Oh, I think I get it now. So the energy gravity put in would just be the amount of energy converted from potential to kinetic, and the energy transferred for friction would just be Ff*Δs.

    I can't think of the effect of normal force... I'm guessing you're implying that the answer for that part is zero joules, then?

    Thank you for your help. (I'd appreciate it if you informed me if I misinterpreted anything, however.)
     
  5. Feb 28, 2014 #4

    Dick

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    You are interpreting everything just fine. As for the normal force, it's called "normal" because it's always "normal" = "perpendicular" to the direction of motion. Can such a force ever transfer energy to or from an object?
     
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