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Homework Help: Relation between speed in water and weight

  1. May 25, 2008 #1
    1. The problem statement, all variables and given/known data
    Any theories about this:
    I was given a lab experience where I had 6 test tubes each with a different weight and I had to time the time it took to get through a 1L cylinder full of water.
    Here is the problem: my data
    3 times out of 6, they all took roughly the same time to fall(±0.03 sec)
    1 out of 6, the heavier the weight, the slower they fell
    2 times out of 6, the heavier the weight, the faster the fall
    I am supposed to find a relation between the speed in water and the weight.

    2. Relevant equations
    speed = distance/time

    3. The attempt at a solution
    Since it's impossible for a heavier weight to be slower than a lighter weight, I rule out that data. I think that water offers more resistance than air to speed so my weight would be falling slower in water than in the air.

    Since these are real world tests, I don't really need to calculate anything. I just need to find the relation between speed in water and weight, with the theories related to the relation. Maybe some equations, if possible. Thanks.
  2. jcsd
  3. May 26, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Weight is a force acting to bring the test tube toward the centre of the earth. There will be a buoyant force from the displaced water, and a drag force of the water acting over the surface area of the test tube. Think about how all these are related and why you might have achieved the results you did.
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