1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conceptual question regarding buoyant force

  1. Nov 21, 2014 #1
    A penguin floats first in a fluid of density p0, then in a fluid of density 0.95p0, and then in a fluid of density 1.1p0. a) Rank the densities according to the magnitude of the buoyant force on the penguin, greatest first. b) Rank the densities according to the amount of fluid displaced by the penguin, greatest first.

    Answer:
    a) All tie. The gravitational force on the penguin is the same.
    b) 0.95p0, p0, 1.1p0.

    Why? Since the penguin displaces different amounts of water in the three fluids(as part b suggests) surely the buoyant force must be different? (Buoyant force = weight of fluid displaced) For example when the penguin is in fluid of density 0.95p0 half of the body is in the liquid, and when it is in the fluid of density 1.1p0 only a quarter of the body is in the liquid, how can the buoyant forces tie?

    Thanks.
     
  2. jcsd
  3. Nov 21, 2014 #2

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Keep track of which forces you're playing with, and you'll be fine.
     
  4. Nov 21, 2014 #3

    Dale

    Staff: Mentor

    Consider Newtons second law. What would happen if the buoyant force were not equal to the weight?
     
  5. Nov 22, 2014 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    What does "amount" mean here? What measure of amount of displaced fluid is relevant for the buoyant force
     
  6. Nov 22, 2014 #5

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Volume. Volume x density x gravity = F
     
  7. Nov 22, 2014 #6

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    No it's not just volume, is it?
     
  8. Nov 22, 2014 #7

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

  9. Nov 22, 2014 #8
    I think I got it:

    For all three fluids, weight of fluid displaced is the same, but

    Weight = mg = (rho)(V)g

    Therefore rho is inversely proportional to V.

    For the fluid of highest density, volume displaced is the least, because just a small volume of this fluid already weights the same as a larger volume of another liquid with a lower density.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Conceptual question regarding buoyant force
Loading...