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Homework Help: Static frictional force

  1. May 9, 2005 #1
    I'm working on the physics behind a Scene in Spiderman, where the Green Goblin is standing on top of a bridge, holding a cable. along that cable is a cable car full of screaming little kids. I calculated the force pulling on the Goblin to be about 23,000lbs...and I'm sure even with his 800% strength increase he couldn't hold on to that.

    So now I'm working on calculating the static friction between the Goblin and the bridge, to see if even if he was strong enough, if he would have enough friction to even stay put. What I have is

    FsMAX (Max force that can be applied before object starts to move) = (Coefficient of static friction) * mass * gravity.

    This comes out to be

    FsMAX = (0.61)(95)(9.8) = 128 lbs

    Where I'm guessing the Goblin's suit's material is aluminum, and the beam he's standing on his metal, and the Goblins weight + suit is 95kg.

    My question is, is this calculation right? Or, would it take more than the 128lb force to start his feet moving because he's actually pulling back using force on the rope? And if that is the case, how would I go about finding the actual FsMAX?
  2. jcsd
  3. May 9, 2005 #2


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    Yep, good job Kirby. The force you calculated assumes a 0.61 coefficient of static friction, and that force is able to resist the cable pulling on the Green dude assuming the cable is pulling horizontally. Of course, if the cable is pulling at some downward angle, there is some increase in static friction because the downward angle gives the Goblin some additional downward force to augment his weight. But assuming a horizontal pull, you've got it figured out. Of course, cartoon characters can even resist gravity if the ground drops out from underneath them... so long as they don't realize it. Maybe some related phenomenon occured to GG...
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