1. Not finding help here? Sign up for a free 30min 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!

What is the magnitude of buoyant force acting on the block?

  1. Jun 28, 2007 #1
    A 0.5kg block of wood is floating in water. What is the magnitude of buoyant force acting on the block?

    I know that Fb = (density)(g)(V)
    The density of water is 1000kg/m3.
    g = 9.8m/s

    There is no way i can calculate the volume of the block with the given information. Also i know the water displaced equals the weight of the block.

    What am i missing to get the buoyant force?
     
  2. jcsd
  3. Jun 28, 2007 #2
    How large does the buoyant force need to be if it has to exactly cancel the gravitational force acting on the block?
     
  4. Jun 28, 2007 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    The fact that the block is floating is crucial!
     
  5. Jun 28, 2007 #4
    If the buoyant force has to cancel gravity then it would be

    Fb = 1000 - (9.81*0.5)
    =995.1
    That looks wrong, i think i need the volume of the block
     
  6. Jun 28, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    That is wrong. Since the block is floating, the buoyant force does cancel gravity, which means Fb equals the force of gravity. But what are you doing subtracting the weight from the density of water??? :bugeye: (Since the units don't even match, such a subtraction has no physical meaning.)

    You don't need the volume of the block. (Hint: It's easier than you think!)
     
  7. Jun 28, 2007 #6
    OK here goes
    Like you said, since the block is floating the buoyant force is equal to the density of the water - the weight of the block
    Fb = 1000kg/m3 - 0.5kg
    =999.5
    That still looks wrong
     
  8. Jun 28, 2007 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Reread what I wrote. I never said such a thing--in fact I said that such a calculation makes no sense!
     
  9. Jun 28, 2007 #8
    That is true, Sorry i misunderstod it but i will go again from the basis of the principle. "Archimedes’ principle states that the buoyant force is equal to the weight of the fluid displaced" so i am now sayin the buoyant force is 0.5N
    ????? I am lost now if that is not the answer. please provide more guidance
     
  10. Jun 28, 2007 #9
    Right, the buoyant force is equal to the weight of the fluid displaced.

    What is the weight of the displaced fluid if the block is floating?

    Hence, what is the buoyant force?

    Note: 'weight' is not the same as 'mass'!
     
  11. Jun 28, 2007 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Actually, what you need here is not Archimedes' principle, but just the conditions for equilibrium. There are only two forces acting on the block:
    its weight, acting down
    the buoyant force, acting up​
    These must balance, since the block is in equilibrium, so the buoyant force just equals the weight (not the mass) of the block. If the mass of the block is 0.5 kg, what's its weight?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?