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Bouyancy, Pressure, Spring Scale

  1. Dec 2, 2007 #1
    1. A 11.4kg block of metal is suspended from a scale and immersed in water. The timensions of the block are 12.7cm x 9.9cm x 9.2cm. The 12.7cm dimension is vertical, and the top of the block is 4.94cm below the surface of the water. The acceleration due to gravity is 9.8m/s^2.

    What is the force exerted by the water on the top of the block? (Take Pinitial=101300Pa. Answer in N.

    What is the force exerted by the water on the bottom of the block? Answer in units of N.

    What is the reading on the spring scale? Answer in N.

    What is the Buoyant Force? Answer in Units of N




    P=F/A where A=area and F=force, P= pressure.



    I used P=F/A for the first one and came up with 4.41N, not the right answer. I believe i found the area on the top of the block... and then used the pressure 101300. That didn't turn out. And then I used the same thing for part 6 ... well actually i don't know because I didn't write it down, but i got 15.745N. That wasn't right either. As for part 7 and 8, I don't see how you can find them out without knowing the density of water, which is 1000 so I used that, and it didn't turn out. Now I'm just stuck :( Help would be very much appreciated! Thanks!
     
  2. jcsd
  3. Dec 2, 2007 #2

    Dick

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    Let's start with the first one. F=P*A is correct. But your answer is way off. What did you get for the area?
     
  4. Dec 2, 2007 #3

    Dick

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    Ok. I see what you did. The pressure on the top of the block is not only due to the 4.94cm of water, you also have to add air pressure, the Pinitial they gave you.
     
  5. Dec 2, 2007 #4
    well for the first question, I forgot to convert to cm. However, I did use F=PxA with the pascals that they gave me, plus the area of the top of the block. Am I supposed to add the Pinitial also?
     
  6. Dec 2, 2007 #5

    Dick

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    Yes.
     
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