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Homework Help: Water tank problem

  1. Mar 11, 2016 #1
    • Member warned not to place multiple questions in the same thread.
    1. The problem statement, all variables and given/known data
    A cylinder water tank its radius 0.5 m, its height is 2 m and it is full of water. Determine the work done to move the water to a height of 4 meters above the height of the cylinder water tank.

    Another question related to it is:
    An uniform iron chain its length is 10 m and its mass is 10 kg for each meter. Calculate the amount of work done to fold it to the top.

    2. Relevant equations
    Work = f *d

    3. The attempt at a solution
    So at the first I thought that plug the casual mgh here because water's mass is not concentrated in a spot. It has different heights. So If the height of the cylinder is 2 meter then I can take the height of 1 meter as a reference. If I sum all the potential energy of the water it will be equal to zero (Since it cancels out). Now I assumed that It is on one level now with a potential energy of 0 and I calculated the work needed by getting the h first which is 1 + 4 = 5 Then using the equation mgh to get the work done.

    Second question: Okay I am facing a problem here.
    So I thought If I want to fold it (That is what I understood of the question and translated it) then I have to move one of its halves to the other. As it is uniform then I should cut it to 5 meters pieces each of them has a mass of 50 kg. Then use the same approach that I used in question 1. To make a level where the potential energy is 0 which is x = 2.5 and take it as my reference point. Then I want it to move upward to 7.5 m. Get the d of these 7.5-2.5 = 5 m
    Plug it into the work equation = mgh but my answer is wrong. In my text book it shows only the final answer without steps. So as I result I found that instead of 50 kg it used 60kg. So I guess I have something wrong here.

    Edit: there is a much easier way to do it.
    Last edited: Mar 11, 2016
  2. jcsd
  3. Mar 11, 2016 #2


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    Science Advisor
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    Gold Member

    I believe you are correct. Half the chain (50kg) is being raised 5m.
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