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Homework Help: Compute the force the a coloumn of water exerts on an object.

  1. Jun 15, 2012 #1
    When a cloudburst in the summer of 2011 in Sweden some sewer pipes were
    completely filled with water. The sewer lids had been fastened, but in some
    places the large amounts of water pushed anyway the sewer lids anyway.

    At one point, the water in a sewer pipe, was pushed up 37.5 m above the level of the sewer lid, in a huge water column.
    The radius of the sewer lid is 29 cm.

    a) Determine the magnitude of the force of the water exerts on the sewer lid

    I know that the answer somehow resolves around computing the pressure in the water column and then find the difference in pressure working respectively upwards and downwards on the lid, and the turn that pressure in to force via the area of the lid.
  2. jcsd
  3. Jun 15, 2012 #2
    deleting my post. Error
    Last edited: Jun 15, 2012
  4. Jun 15, 2012 #3
    But if the water column persists, would there be some kind of force that will keep on working?

    It has been given as a national exam a couple of years ago, so I do think that it must be possible to find a solution!
  5. Jun 15, 2012 #4
    And btw, there is a little drawing included.

    Attached Files:

  6. Jun 15, 2012 #5


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    Your picture makes a good point- the column of water will rise to exactly the height it reaches on the hill side. And the force pushing it up is simply the weight of the water itself. Multiply the height of the water by the cross sectional area and the density of water.
  7. Jun 15, 2012 #6
    Thanks so much for your responses guys

    But is that the force working ON the sewer lid?

  8. Jun 15, 2012 #7
    I think you forgot the gravitational acceleration, g..

    Respecting Pascal's law, yes it is. :smile:
  9. Jun 15, 2012 #8
    Okay ... So the whole force excerting on the lid is given by

    Fwater-on-sewer = ρwater * hcolumn * Asewer * g

  10. Jun 15, 2012 #9
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