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Homework Help: Energy Problem

  1. Jan 7, 2004 #1
    A 8.63 kg box slides down a long, frictionless incline of angle 20 degrees. It starts from rest at time t = 0 at the top of the incline a height 24.6 m above ground.
    Find the potential and kinetic energy of the box at t = 1.8s. Answer in units of J.

    acceleration is gsin20
    so I got velocity
    by gsin20 x 1.8 seconds
    and then I used .5mv^2 to find KE
    and then for PE
    I found PE at the highest point
    which is mgh
    and then subtracted KE from that

    I got 1925.2569 J for PE and 157.3863974J for KE (ignore sig figs)
    I entered this into UTexas and got it wrong.
    Please help.
  2. jcsd
  3. Jan 7, 2004 #2
    Maybe I've had too much to drink this eveing but I can't see where you went wrong.
  4. Jan 7, 2004 #3
    Yeah neither can I
  5. Jan 7, 2004 #4
    Where is this UTexas thing?

  6. Jan 7, 2004 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Your method is fine. I assume you are entering your answer into some online system? If so, they can be picky; it might not tell you you are correct unless you give exactly the answer it expects. Try varying the number of significant figures you enter. (I hope you didn't enter all 8-10 "significant" figures!)
  7. Jan 7, 2004 #6
    This UTexas thing is at hw.utexas.edu. It's an online program that my physics teacher uses to torture us. Doc Al, the program specifically says "Enter what your calculator says" and it allows a 1% error anyways so I'm pretty sure that's not the problem.
  8. Jan 7, 2004 #7
    It's always possible that the program might contain an error. Have you asked your teacher about this problem?
  9. Jan 7, 2004 #8
    Heh...we've been snowed in for 2 days...problem has some time to fester...
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