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!

Homework Help: Buoyant force in seawater and freshwater

  1. Jan 5, 2013 #1
    1. The problem statement, all variables and given/known data
    I'm doing a problem that is likely a common one. It is comparing the buoyant force of an object in seawater to that of the same object in freshwater. which has more buoyant force acting on it?

    2. Relevant equations

    not sure of equations at this stage, and we are just doing it real simple so we are assuming all things are equal other than the water density.

    3. The attempt at a solution
    My take on it is that the object will displace more volume of freshwater than seawater and the amount displaced is determined by the volume of the object.

    However if we captured the displaced water and weighed it they would have the same mass but different volumes.

    I believe the buoyant force would be the same, just different volumes of the water has been displaced dependent on the water type.

    Further to this, if the object was a boat for example, the mass of the water displaced would be the same as the mass of the boat.

    Have I interpreted the basics of buoyant force correctly or am I off down the wrong path ?
  2. jcsd
  3. Jan 5, 2013 #2
    Hi urbano
    The buoyant force is determined by the weight of the displaced liquid.
    Not its volume.
    Of course, the weight is related to the volume, but what matters is the weight.
    So if you submerge o solid of some volume V in two different liquids (seawater or freshwater), you will displace the same volume V of said liquid.
    but since those liquids have different densities, for one of them, more mass, will be displaced, and the buoyancy resulting force will be stronger.

  4. Jan 5, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    OK with the added comment above in red.. The assumption is that the boat is floating. All of the above would be incorrect if the boat sank. However, irrespective of floating or sunk, it is always true that the buoyancy force is equal to the weight of the displaced water.
    Last edited: Jan 5, 2013
  5. Jan 6, 2013 #4
    ohh I'm definitely confused....

    oli4 makes mention of the fact that if I submerge an object in freshwater or seawater then the volume displaced will be identical. I believe this is due to the fact that the object has been submerged.

    I guess it's different in the case of a boat floating ? In the case of a floating boat , I believe the boat would displace different volumes of seawater compared to freshwater (i.e less of the boat is physically in the seawater), but the mass of the displaced waters (sea and fresh) would be equal.

    ??? is this correct thinking??
  6. Jan 6, 2013 #5
    Your thinking is correct.
    Let us say that an object is submerged to a volume V in a given fluid . Then the fluid applies a force on the object ,which is equal to the force on an equal volume V of the fluid due to the rest of the fluid body. This force is equal and opposite to the weight of the fluid of volume V ( assuming that the fluid body is at rest). This is the bouyant force.

    Since the boat is floating , the net force on it is zero ( in the vertical direction) . What are the forces? The earth's gravitational pull (i.e weight of the boat) and the bouyant force of the fluid. Since the weight of the boat is same in both cases the bouyant Force is also same in both cases.
    What is the bouyant force ? It is equal to the weight of the displaced fluid , which is equal to the (density of fluid)*(volume of fluid displaced ) . Thus for the fluids with greater density ,lesser volume is displaced , and hence the volume submerged would be less.
  7. Jan 6, 2013 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    All you just said is corrrect, so why are you " definitely confused"?? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook