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Friction and Work (check please)

  1. Jun 7, 2016 #1
    1. The problem statement, all variables and given/known data
    A 25 kg bear slides, from rest, 8 m down a lodgepole pine tree, moving with a speed of 6.4 m/s just before hitting the ground

    (a) What change occurs in the gravitational potential energy of the bear-Earth system during the slide?
    W = F*D
    W = (m*a)*D
    W = 1960

    (b) What is the kinetic energy of the bear just before hitting the ground?
    KE = 0.5*m*v^2
    KE = 512

    (c) What is the average frictional force that acts on the bear?
    Ei = m*g*h = 1960
    Ef = 0.5*m*v^2

    W = -Fk*h
    Ei + W = Ef
    1960 + (-Fk*h) = 512
    -Fk = 120.67

    Could someone check to see if i solved this correctly?
  2. jcsd
  3. Jun 7, 2016 #2
    The process appears to be correct (didn't check your arithmetic) . There seems to be an extra minus sign before [itex] F_k [/itex] in the last line ?!
  4. Jun 7, 2016 #3
    So...just check the numbers?
  5. Jun 7, 2016 #4
    You could do that with a calculator, the last one may be wrong though , incidentally did you take the downward direction as -ve?
  6. Jun 7, 2016 #5
    what does -ve stand for?
  7. Jun 7, 2016 #6
  8. Jun 7, 2016 #7
    yes, should i have?
  9. Jun 7, 2016 #8
    You can take it whatever you want. Your method is correct then , there is no extra negative sign. Do check the arithmetic for c) I suspect it's wrong.
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