Buoyant Force

  1. A completely submerged body displaces a volume of liquid equal to its own volume. The buoyant force equals the weight of the fluid displaced.
  2. When an object weights less than the total volume of fluid it can displace, it will settle down until the buoyant force equals the weight and it floats partially submerged.

This java applet shows effect of buoyant force acting on an object (less dense than water).

  1. You can change the density of the floating object (square block)
  2. click the mouse near the top of the gray area within top left block.
    drag the mouse up and down to change the density.
  3. Another block on the right has the same amount of gray area
    1. this gray area will be redistributed uniformly.
    2. (Same mass distributed over larger volume --> less density)
    3. the block starts moving down until the buoyant force equals its weight .
    4. the volume displaced is represented by the gray area at the lower left corner.
    5. the curve for pressure varies with depth is shown on the right side.
  4. When the block is in equilibrium,
  5. you can click within the block and drag it up and down.
    the difference between the gravitation force and the buoyant force cause the block to move sinusoidally. (SHM)
  6. During the animation:
  7. press the mouse button to suspend the animation. (release the mouse button to resume)

Your suggestions are highly appreciated! Please click hwang@phy03.phy.ntnu.edu.tw

AuthorĄGFu-Kwun Hwang, Dept. of physics, National Taiwan Normal University
Last modifiedĄG 

Subject: interactive bouyancy demo 
Date:  Thu, 3 Oct 2002 15:47:15 +0100
From:  "porterl" 

I like the basic idea, it is clear what is going on and is easy to adjust the variable.
I like the added clarity of levelling the two liquid levels in the bottom diagrams.

I am an retired Physics teacher ( always teaching in English Grammar schools, with pupils 11 to 18 years), during my time I would have given a lot for these interactive computer programs. I think I would have used them as revision aids rather that as primary teaching aids.

As a variation to your idea :-

I often got my students to hang a metal rod from a force meter, record it's weight . Then record the meter reading as the rod was lowered into water, also recording the depth of immersion. A graph would be drawn.

After discussion of the forces involved and exactly what force the meter was recording, the experiment was repeated but this time with a candle of the same dimensions as the metal rod . Most pupils found the reading of zero when the candle floated to be not what they expected..

Most straight line graphs produced by pupils in elementary Physics go through the 'origin' but the candle's one does not.

I am just in school for today and tomorrow to replace a teacher who is away.

                                        Mr.  Leslie  Porter.
P.S. Going home now to fly my kites, well when you are retired you can actually spend time on your hobbies. L.P.