2B40.10-Weigh Submerged Block – Physics Department

Description: This demonstration illustrates that the buoyant force on an object is equal to the weight of the water displaced.

Equipment:

Setup Procedure:

Pour water into a Pyrex beaker and add food coloring. Stir using the glass rod.
Place the other Pyrex beaker on the weight scale and zero the scale (accounts for mass of the beaker).
Connect the rods as shown in the picture to hold the newton scale.
Make sure the apparatus is lined up with the newton scale, so the professor can just lower the scale.

Demonstration Procedure:

Close the valve on the apparatus.
Pour the colored water into the glass parat of the apparatus.
Weight the weight of the desired mass and record it.
On the side of the apparatus, there is a small water reservoir. Move the metal guide to the water surface for a reference mark (see fourth picture).
With the weight on the newton scale, slowly lower it into the water by moving the newton scale down the post.
Record the new weight of the mass in the water.
With the empty Pyrex beaker under the drain, open the valve to let out some water. You want to do this so the water gets to the point where it was before the mass was lowered into the water (this is when the maker you set is helpful). Note that you have to periodically close the valve to get the true water level reading.
When the water reaches the pre-weight level, close the valve and weigh the water displaced on the force scale.
The weight of the water displaced should equal the buoyant force (the difference in weight you recorded).
When finished, please empty the water out into the Pyrex beakers.

Example:

In the example in the picture, we used a 1kg mass. The dry weight was 9.8N, the submerged weight was about 8.5N (second picture). The mass of the water was 125g (third picture), so the weight was 1.25N.
9.8N – 1.25N = 8.55N, so the buoyant force was 1.25N.

Note: The apparatus is old and leaks slowly.