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Homework Help: Dynamics Question

  1. Dec 8, 2007 #1
    1. The problem statement, all variables and given/known data

    Two buckets of nails are hung one above the other and are pulled up to a roof by a rope. Each bucket has a mass of 5.0 kg. The tension in the rope connecting the buckets is 60 N. Calculate the acceleration of the buckets.

    2. Relevant equations


    3. The attempt at a solution

    The first thing I did was add up the two masses of the two buckets to get 10.0 kg. Then, since F(net)=F(A)+F(g), I substituted to get ma=F(A)+mg. I then rearranged for acceleration to get a=F(A)+mg/m. I plugged in the values for a=60 N+(10.0 kg)(-9.81 m/s^2)/10 kg to get 3.8 m/s^2 [down]. The answer is 2.2 m/s^2 [up]. I did notice that if I were to use a value of 5 kg (the mass of a single bucket), I would get the right answer. I suppose my error may have something to do with the fact that the tension is between the buckets, but we have learned nothing about that yet and I might need a bit of information having to do with that.

    Any help is greatly appreciated.
  2. jcsd
  3. Dec 9, 2007 #2
    Yeah, thats the trick. Since they tell you the tension is in the rope between the buckets, this force only acts on the bottom bucket-- hence, when you do your calculations, you can only include the bottom bucket. If you wanted to do it for both buckets, you would need to know the tension in the rope above the first bucket, which isn't given to you (although, if you wanted to, you could find it). In any event, the acceleration of both buckets will be the same, so you really don't care after finding the acceleration of the bottom bucket.

    The mass of the top bucket will have no impact on the tension in the rope between them, so you don't want to include that mass in your acceleration calculation.
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