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Centripetal Acceleration Question

  1. Feb 8, 2014 #1
    An object weighing 4 newtons swings on the end of a string as a simple pendulum. At the bottom the swing, the tension in the string is 6 newtons. What is the magnitude of the centripetal acceleration of the object at the bottom of the swing.


    Centripetal Acc. = v^2/r Sum of forces = T+Ac=mg???


    Attempt

    -T-mg=Ac?
    -6-4 = Ac
    I don't know where to go from here.
     
  2. jcsd
  3. Feb 8, 2014 #2

    vela

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    Newton's second law says
    $$\sum \vec{F}_i = m\vec{a}.$$ The centripetal acceleration is not a force. It's an acceleration. It goes into the righthand side of F=ma. The tension and the weight go into the lefthand side. Try again and pay attention to the sign of T and mg when summing the forces.
     
  4. Feb 8, 2014 #3
    So I set it up as Sum of forces = T-mg=Ac
    6-4=Ac
    2?
     
  5. Feb 8, 2014 #4

    vela

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    Closer. The lefthand side is correct, but the righthand side isn't. You can't add up a bunch of forces and then set the result to something that isn't a force. ##a_c## is the centripetal acceleration; it's not a force. It's the ##a## in ##ma## on the righthand side.
     
  6. Feb 8, 2014 #5
    ah my bad I was correcting it as you answered .
    so is it 2=mAc
    so its 2 g ? the answer choices only come in a number times g
     
  7. Feb 8, 2014 #6

    vela

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    Not quite. You need to figure out what the mass of the object is from its weight.
     
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