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Homework Help: Tension and Friction with Acceleration!

  1. Sep 16, 2013 #1
    1. The problem statement, all variables and given/known data

    A building is on fire and the occupant of a room ties together some bed sheets, attaches them to something immovable in the room and throws the bedsheet ‘rope’ out the window. The person weighs 800 N and the bed sheets can only hold a force of 600 N before they break. If the person descends (starting with zero vertical velocity) 12 m (about 4 stories) using this bedsheet rope, applying just the right amount of friction on the rope to provide maximum braking without breaking the rope, how fast is the person going after 12 m of descent? Give you answer in miles per hour so you have a feel for how fast this is. (26.8 m/s = 60 mph)

    2. Relevant equations

    I'm sorry, I do not know how to use the LaTex Math Typesetting, but the equations are pretty basic.

    Fnet = ma

    Fnet = F1 + F2 + F3 + ... + Fn

    Vf^2 - Vo^2 = 2ay

    3. The attempt at a solution

    Okay, so what I did is say that the person must be using a maximum downward force of 600N on the rope so it doesn't break. This makes then friction equal to 200N, since it acts in the opposite direction of the 800N downward of the occupant.
    So, (-600N)/(9.8kg*m/s^2) = -7.35 m/s^2. This is person's acceleration downwards.
    Plugging this into the final equation yields Vf = √(2*(-7.35 m/s^2)*(-12m)) = - 13.3 m/s or -29.7 mph.

    HOWEVER, my classmate says that the rope pulls upward with 600N and since the rope is applying the force of friction, the there're 600N of friction. So that would make the net force -200N. Redoing the calculation gets -17 mph.

    So, which is correct? My classmate's makes sense, but it seems off to me...so any input is very, very appreciated! We could also both be wrong haha. Thank you for taking your time!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 16, 2013 #2
    If the rope exerts some amount of friction, then by Newton's third law it is itself subject to the same amount of force (acting in the opposite direction).
  4. Sep 16, 2013 #3
    So, then the friction must be equal to the tension limit of 600N?
  5. Sep 16, 2013 #4
    It is in the interest of the occupant to use the max friction afforded by the rope.
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