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Homework Help: Tough Question: Bouyancy and 1-D Motion

  1. Jan 22, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball whose density is 0.4*103 kg/m3 falls into water from a height of 9cm. To what depth does the ball sink? Only consider bouyancy and ignore retardation due to viscocity.

    (a) 9cm (b) 6cm (c) 4.5cm (d) 2.25cm

    2. Relevant equations

    Bouyant force = Volume of Body*Density of Liquid*Gravitational Acceleration

    v2 = u2 + 2as

    F = ma

    Density = Mass / Volume

    3. The attempt at a solution

    Well, first I figured out the final velocity with which the body entered into the water using the third equation of motion (second equation stated above).

    Then I figured out the retarding force acting on the body because of the bouyancy offered by the water (using the second, the first, and then the third equation stated above).

    Finally I plugged in the retardation offered by bouyancy into the third equation of motion, and figured out the final answer. My answer was 3.6cm, but that is nowere in the answer choices.

    Please help.
  2. jcsd
  3. Jan 22, 2010 #2
    In the retardation equation, did you include the fact that the ball's weight is still acting downwards on it, and that the retardation is caused by a resultant upwards force equal to buoyancy minus weight?
  4. Jan 22, 2010 #3
    Umm.... no I didn't. I'm stuipid, right?

    Yup, well that solved the problem.

    Thanks a lot.
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