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Homework Help: How to find the weight of an object given the force

  1. Oct 24, 2012 #1
    1. The problem statement, all variables and given/known data
    An old, heavy wooden door leads to a basement storm cellar. If it requires 400N to lift the door by its handle and swing it open, what is the Weight (in N) of the door itself? (Also, there is a picture that shows from the hinge to the handle is 1.6m)

    2. Relevant equations
    F=ma ?

    3. The attempt at a solution
    I usually get stuck on the easiest of problems because I over think them. Would I just use the F=ma formula? Plugging it in as: 400N=m(10m/s)? (My teacher allows us to use 10 instead of 9.8 for acceleration). If so, the mass would come out to be 40kg which in turn is 400N.

    I would appreciate the help if I did this wrong. Thanks!
  2. jcsd
  3. Oct 24, 2012 #2
    Are you familiar with momentum?
  4. Oct 24, 2012 #3
    My mistake, acceleration is m/s^2. It's velocity that is just m/s which is included in calculating momentum.. P=mv. Do I use this formula somehow?
  5. Oct 25, 2012 #4


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    You should consider taking moments of forces about the hinge and assume that the door is initially raised very slowly and is in equilibrium ( not accelerating) during the initial force application. First identify the forces acting on the door before applying the torque equilibrium equation.
  6. Oct 25, 2012 #5
    PhanthomJay is right. It would help to draw a free body diagram. You will also have to either know where the CG of the door is, or assume it to be in the center of the door. If that be so, the weight will work out to twice that required to open it. (Drawing the FBD and working the equations in my head. You might want to check me by doing it on paper.)
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