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Conservation of Energy question

  1. Nov 26, 2008 #1
    A 28 kg bear slides, from rest, 6 m down a lodgepole pine tree, moving with a speed of 5.9 m/s just before hitting the ground.

    (a) What change occurs in the gravitational potential energy of the bear-Earth system during the slide?
    (b) What is the kinetic energy of the bear just before hitting the ground?
    (c) What is the average frictional force that acts on the bear?

    To start off with i set up an energy bar chart and i know that it begins with gravitational energy and ends with kinetic energy. The kinetic energy is Ek=1/2mv^2 so thats the answer for b. I'm not sure how to find the frictional force on the bear though, and i think i need that before i can figure out the change in the gravitational energy. I think that the frictional force would equal the product of the kinetic friction coefficient and the normal force. But i keep coming up with the wrong answers so i'm not sure.
  2. jcsd
  3. Nov 26, 2008 #2


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    No, the change in the gravitational Potential Energy is its PE at the bottom of the slide minus its PE at the top of the slide. In calculating PE, you can use any convenient reference point.
    Yes, but neither is given. Instead, either use Newton 2 or conservation of total energy.
  4. Nov 26, 2008 #3
    Okay well i would have to use F=ma to find the frictional force right? I'm getting confused because i don't know the acceleration, should i just assume constant?
  5. Nov 26, 2008 #4
    Okay, i'm thinking maybe i should just scratch using N2L, what if i just use the equation for gravitational energy, so Eg=(28kg)(9.8m/s)(6m) which gives me 1646.4. Then to find the change i would subtract the kinetic energy of the bear at the bottom of the slide to give me the change in gravitational energy?
  6. Nov 26, 2008 #5
    Okay, lol that was it. I think the main problem was i was thinking that this tree was like at an incline, like he was sliding down it like a slide. Yeah i deffinately overthought this one.
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