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Energy/Work Problem: Friction of sliding down pole

  1. Jul 18, 2017 #1
    1. The problem statement, all variables and given/known data
    A fireman of mass m slides a distance d down a pole. He starts from rest. He moves as fast at the bottom as if he had stepped off a platform a distance h≤d above the ground and descended with negligible air resistance.


    What average friction force did the fireman exert on the pole?
    Express your answer in terms of the variables m, d, h, and appropriate constants.

    2. Relevant equations
    U=mgh
    K= 0.5mv^2
    K1+U1+W(other)=K2+U2
    W=Fs

    3. The attempt at a solution
    Since friction is a nonconservative force, I figure I'm supposed to use K1+U1+W(other)=K2+U2 and find the W(other), and then use the equation W=Fs to find the actual force of the friction.

    When the fireman is at rest at height, d, he has no KE, just PE, which is:

    U1= mgd

    As he reaches the ground, some of the potential energy was converted to kinetic energy, but the rest of it was lost through friction:

    mgd+W(other)=K2=0.5mv^2

    To find W(other) I need to find the velocity of the fireman. This is where I start to get unsure. The problem states that his velocity is constant at all heights (h) after he starts sliding down from his initial position, d. I decided to set up another conservation equation, in which he starts at some height, h, and then reaches the ground:

    mgh=0.5mv^2 - W(other2)

    Since W=Fs, and s=h, I substituted:

    mgh=0.5mv^2 - Fh

    Where F is the friction force I want to find out. I solved for v^2, getting:

    v^2= (2/m) * (mgh + Fh)

    I plugged this back into the conservation equation for the entire distance, getting:

    W(other)=mgd-0.5m[(2/m) * (mgh + Fh)]=mgd-mgh+Fh

    W(other) is also equal to a force over a distance, so:

    mgd-mgh+Fh=W=Fd

    Isolating F:

    mg(d-h)=Fd-Fh=F(d-h)

    Then the (d-h) terms cancel, getting mg=F. However, this is incorrect, and the problem states that the answer has the term d in it.

    Can anyone point out where my logic and/or math was wrong?
     
  2. jcsd
  3. Jul 18, 2017 #2

    Doc Al

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    Staff: Mentor

    You went off the rails (a bit) when you introduced the speed "v". Instead, just express U2+ K2 in terms of the given variables.

    Also, the work done by friction is negative, so be careful with signs.
     
  4. Jul 18, 2017 #3
    I'm not really sure how to express U2+K2 in terms of the given variables d, h, and m.

    I'm at mgd = U1 and K1=0.

    So mgd + 0 + W(other) = U2 + K2.

    Can I equate W(other)=F*s, making the equation:

    mgd + F*d = U2 + K2 ?

    If U2 equals zero (since the fireman is on the ground), that leaves:

    mgd + F*d = K2.

    But I still don't know K2 or F. Any hints on where I'm supposed to get my second equation for a system of equations? I haven't used the "He moves as fast at the bottom as if he had stepped off a platform a distance h≤d above the ground and descended with negligible air resistance." part of the problem yet.
     
  5. Jul 18, 2017 #4

    haruspex

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    Gold Member
    2016 Award

    Good. Now find the corresponding equation for the case where there is no friction and the starting height is h.
     
  6. Jul 18, 2017 #5
    Would that be as simple as:

    mgh = K2

    Since there is no initial kinetic energy nor final potential energy?

    Since speed is the same at the bottom regardless of air resistance (according to the problem), would I just set these two equations equal to each other and solve for Fk?
     
  7. Jul 18, 2017 #6
    I just submitted the answer and this was correct. Thanks for the help everyone!
     
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