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Homework Help: Problem calculating speed at the top of a loop-the-loop

  1. Jul 20, 2012 #1
    This is the problem:
    A 15.0 kg box is slid down a 6.00 meter high ramp to pick up speed to do a vertical loop-the-loop whose radius is 2.00 meters. All surfaces are conveniently frictionless.

    Part A) asks to find the Speed at the top of the loop-the-loop.
    Part B) asks to find the Normal force on the box at the top of it's motion.

    I'm just not sure I've solved it correctly.
    Here's what I did it:
    PEi + KEi = PEf + KEf
    (1/2)*m*vi2 + m*hi*g = (1/2)*m*vf2 + m*hf*g
    0 + 6mg = (1/2)*m*vf2 + 4*m*g
    6g = vf2/2 + 4g
    vf = sqrt(4g) = 6.26m/s

    B) Normal = Centripedal Force - mg = m*v2/r - mg = 15*(6.262/2) - 15*9.8 = 117.6 N
  2. jcsd
  3. Jul 20, 2012 #2
    The work in part (a) and (b) is good! Can you check your calculator result in part (b) though? I calculate a different number: [itex]\frac{(15kg)(6.26\frac{m}{s})^2}{(2m)}-(15kg)(9.8\frac{m}{s^2}) = 147N[/itex]
  4. Jul 20, 2012 #3
    I see my mistake. I've divided in the wrong place.
    Redoing it made get to 147N :D
    Thanks for the help!
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