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Homework Help: Find mass of an object hanging on string

  1. Nov 22, 2011 #1
    please tell me how to find the mass of an object which is hang on the string???
    I mean what would be the formula ????
    i am also writing here my numerical question....
    Q:two bodies A and B are attached to a string which passes over a pulley so that two bodies hang vertically.if the mass of body A is 4.8 kg find the mass of body B which moves down with an acceleration of 0.2m/s.s.(g=9.8m/s.s)
  2. jcsd
  3. Nov 22, 2011 #2
    Make a diagram of the situation.

    Look carefully at mass B. How many forces act on mass B?
  4. Nov 22, 2011 #3
    no force is acting on mass B the formula given in our text book is:
    α=(mB-mA/mB+mA) x g

    i dont know how to arrange it to find the mass B......:confused:
  5. Nov 22, 2011 #4
    We are trying to show HOW to get that formula.

    Look at mass B. It has the Earth pulling it down - that is the weight mB x g of mass b - and the string pulling it up by a tension T.

    Now use Fnet = ma for mass B.
  6. Nov 23, 2011 #5
    Fnet = ma ............for mass B

    mBg - T = mBa

    Fnet = ma .............for mass A

    T - mAg = mAa

    If the tension T is eliminated from the above two equations then one gets the equation required.
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