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Question on energy

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data
    A .444kg block, slides down a .888m ramp, at an angle of 33.3°. The block starts from rest, and reaches a speed of 2.22 m/s. How much energy is lost?


    2. Relevant equations
    If I'm thinking correctly, I'd use the work equation...
    W=ΔK+ΔUg+ΔUs
    ΔK=(1/2)m*vf^(2) - (1/2)m*vi^(2) (The second part of this drops out, because the initial velocity is 0)
    Oh, and there's no spring involved, so ΔUs drops out.


    3. The attempt at a solution
    So, the stated equation can be re-written as...
    (1/2)m*vf^2 +m*g*Δy (ΔY=Δxsin∅)
    Then...
    (1/2)(.444kg)(2.22^(2)m/s^(2)) + (.444kg)(9.80m/s^2)(.888m sin(33.3°))

    The answer I get, assuming I'm right is 3.22 J? Am I getting anywhere with this, or am I completely off?
     
  2. jcsd
  3. Oct 24, 2011 #2
    Hi Timebomb,

    can you clear up something in the question for me? When the question asks "how much energy is lost", what does that mean?
     
  4. Oct 24, 2011 #3
    I don't know. That's why I'm asking. That's exactly what the question says, on my assignment.
     
  5. Oct 24, 2011 #4
    So if your answer is around 3 Joules, where is that energy going? Through what process is the energy lost?
     
  6. Oct 24, 2011 #5
    Oh I see what the question is saying. Okay, I also see your mistake. your potential energy is the maximum energy that you had when the block was at rest. So you shouldn't be adding the kinetic energy to it, you should be subtracting the kinetic energy from it.

    Basically the energy lost is equal to all of the potential energy that didn't get converted to kinetic energy.
     
  7. Oct 24, 2011 #6
    Maybe the 3.2 Joules is the energy lost/given off, by the block sliding down the .888m ramp? If that isn't right, then I don't know what is.
     
  8. Oct 24, 2011 #7
    Yeah, you're right, it must be friction. did you read my other post?
     
  9. Oct 24, 2011 #8
    Yea, it's the friction. Because the next part of the problems says, "If the energy is lost to frictional heating, what is the magnitude and direction of the frictional force." I'm just curious if the number I got was right.
     
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