Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Hydrostatics/Equilibrium: An object hanging by a light cord in a bucket of water

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data
    A rock with a mass of 2.8 kg is suspended from the roof of an elevator by a light cord. The rock is totally immersed in a bucket of water that sits on the floor of the elevator, but the rock doesn't touch the bottom or sides of the bucket.
    When the elevator is at rest the tension of the cord is 19.9 N. Calculate the volume of the rock in cubic centimeters (cm^3).

    2. Relevant equations
    Pascal's Law: P = Po + gρh
    V = Ah or V = m/ρ
    P = F/A
    ΣF = 0

    3. The attempt at a solution
    I am a little confused about how to go about this problem as I'm just starting into fluid dynamics... but my initial step into this was to assume that if the rock is not touching the bottom of the bucket, and is also not floating up, that it is in a state of equilibrium.

    So I started by trying to recognize all forces acting upon the rock: T-tension on the cord upwards, mg-weight of the rock due to gravity, PoA- force of the Atmospheric pressure acting down on the rock, and PA-pressure of the water acting up on it. From this i stated that:
    T + PA - mg - PoA = 0 thus,
    T + PA = mg - PoA

    However since i know the mass of the rock, tension of the cord, and i'm assuming gravity to be 9.81 m/s2, this equation can be simplified as far as:
    A(P-Po) = 7.56N

    Now the Po will equal just the standard atmospheric pressure of 101.345 kPa, but that still leaves 2 unknowns that i'm not too sure how to deal with.

    Since i am looking for Volume of the rock, i can relate volume in only two equations i can think of, which are V = Ah, and V = m/density. Since im already working with a formula that involves the area, im thinking my next step would be to somehow use pascal's law to find the depth the rock is submerged in to find the volume at the end?

    Anyways, any tips would be nice. Let me know if i'm on the right track, or if i'm way overlooking a simpler solution.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 22, 2009 #2


    User Avatar
    Homework Helper

    You have to use Archimedes' Principle, not Pascacl's law.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook