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Homework Help: Pressure Question of swimmer

  1. Jan 9, 2010 #1
    Alright this may be a simple and stupid question but I'm still not sure of what the answer might be :( So I hope you guys can help me out a little. =]

    A swimmer dives off a raft in a pool. Does the raft rise or sink in the water? What happens to the water level in the pool? Give reasons for your answer.

    Attempt to find an answer:
    So, the raft was already floating in the water with the swimmer on it. Meaning the total mass is less than or equal to the upthrust due to atmospheric pressure. If the swimmer jumps off of the raft then there would be less force to act against the upthrust therefore the raft will rise. However, for the second part I am unsure.

    At first, I thought that the water level would reduce because less water is being displaced. But then for some reason I thought that the water level might be the same? Because the swimmer is not diving off out of the pool but he is still in the pool. I don't know I'm quite confused with this part of the question.
  2. jcsd
  3. Jan 9, 2010 #2


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    Homework Helper

    For the second part: remember that the amount of water displaced is equal to the submerged volume of anything in the water. So for example, if the swimmer is completely underwater, the amount of water he's displacing is equal to his volume. The amount of water the raft is displacing is some fraction of the raft's total volume. This is useful because the more water is being displaced, the higher the water level.

    You know, I presume, that the buoyant force on any object is equal to the weight of the water it displaces. What's the relationship between the total buoyant force on all objects in the pool and the water level?

    What can you figure out about the total buoyant force on all objects (swimmer and raft), before the swimmer jumps vs. after he jumps?
  4. Jan 10, 2010 #3
    That the buoyancy force is equal or more than the force exerted by the swimmer and the raft together and afterwards when the swimmer jumps off and only the raft floats and the swimmer is submerged into the water when he jumps off? :S
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