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Homework Help: Force on door from two hinges - easy - help anyone?

  1. Nov 14, 2004 #1
    torque/force problem - two door hinges - help anyone?

    A large door of height - 2.1 m - along y axis that extends vertically upward and width - 0.95 m - along x axis that extends outward from the hinged edge of the door. A hinge at 0.30 m from the top and a hinge at 0.30 m from the bottom, each support half the door's mass which is 28 kg.

    (a) In unit-vector notation, what is the force on the door at the top hinge?
    ___ N i +____ N j
    (b) In unit-vector notation, what is the force on the door at the bottom hinge? ___ N i +____ N j


    I don't understand where the horizontal force - vector in i direction - is coming from if there's no horizontal movement and only gravity is acting along the y-axis or j hat

    so my answers for the j hat vectors are simply 9.8x14= 137.2N which is the correct answer... but i don't understand parts A and C???

    help anyone???
    Last edited: Nov 14, 2004
  2. jcsd
  3. Nov 14, 2004 #2
    help please~!

    anyone out there????? i don't need to give me the answer... could anyone just point me in the right direction???? how to find the i-hat vector forces in this problem?????

    thanksss a lot
  4. Nov 14, 2004 #3
    Take the moment about the bottom hinge to find the answer for a) and then swap to the top hinge to find b)
  5. Nov 14, 2004 #4
    i, too, am stuck on this problem, and what do you mean by take the moment? im a bit rusty, and trying to remember that, but cant seem to remember...... any help?
  6. Nov 14, 2004 #5
    First you draw the free body diagram. Putting in all the forces into the diagram, there are altogether 5 forces, 2 at each hinge and the door's self weight. There are 2 unknown forces in the diagram since you have been told that the vertical component at each hinge is half of the door weight. 2 unknown forces and 3 equations derived from the diagram, it can be solved.
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