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Homework Help: Forces and angles

  1. Feb 9, 2005 #1


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    hi, i need help on a question

    its like... there is a operator supported a plank horizontally by its two ends, but when loaded bricks on its centre and found that the weight of 166kg of bricks was just enough to break the plank. But an identical plank is now being used as a ramp to load a full 200L fuel drum onto a truck. The drum and its contents have a mass of 197kg.

    a) Carefully explain why the operator could still use this plank to load the fuel drum.
    b) what is the minimum angle to the horizontal at which the plank can still used to load the drum?

    i don't know where to start... as i don't know how to answer part a) :mad:
  2. jcsd
  3. Feb 9, 2005 #2
    min angle would be = cos-1(166/197)
  4. Feb 9, 2005 #3
    and the reason the greater weight could be carried is b/c when the plank is at angle the internal moment arm is less
  5. Feb 9, 2005 #4


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    The original plank broke at 166 kg because the gravitational force NORMAL to its surface exceeded the plank's capacity. When the plank is tilted, the component of gravitational force NORMAL to the surface is reduced by a factor of cos(A), where "A" is the plank's tilt angle with respect to horizontal ground. Thus, the minimum "A" required for 197 kg can be determined from:
    (197)*cos(A) = (166) <--- (derive this yourself by resolving grav force into components normal and parallel to plank surface)
    Last edited: Feb 9, 2005
  6. Feb 9, 2005 #5
    how do i attach a document
  7. Feb 9, 2005 #6

    this should help

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