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 Buoyancy: Definition-Facts-Math-Fun Science SuperSchool Experiment

Buoyancy is the upward force of a gas or a liquid or any fluid that lifts an object and seems to decrease its weight. Buoyancy was discovered by Archimedes in the third century BC and is a principle of physics to know, learn and experiment with.  Learn about buoyancy and do some buoyancy experiments here and now.

Buoyancy in Liquid or Air – Simple Balloon Experiments

Imagine two balloons, one filled with air and another balloon filled with helium. The helium balloon must be held and tied to prevent it from escaping high into the air. If a helium balloon is released, it keeps rising because the pressure of the air pushing upward is greater than the mass of the balloon and the helium within. The balloon stops rising when it reaches a height where the upward force is equal to the downward forces of gravity and air pressure.  Compare next an air-filled balloon to the helium balloon. The air-filled balloon will sink in the air if you let it go. In simple terms the air balloon falls because the combined total downward force exerted by the rubber and the air within the balloon is greater than the upward force of the surrounding air. This balloon comparison and experiment is something to think about and remember.

In another experiment, if the same air- or helium-filled balloon is immersed in a bucket of water filled to the very rim, and then the balloon is gently pushed downward, the volume of the air in the balloon will displace the exact same volume of water from the bucket and this will cause the water to overflow. There is an actual upward force of the water that is equal to the volume of the water displaced by the balloon. If that amount of overflow water is measured and caught in a pan or basin, then the volume of the balloon can be determined. The volume of the gas in the balloon can be measured based on the formula 4/3 x (3.14, the pi value) x radius cubed (r x r x r). 

Buoyancy Experiment Materials and Methods

For this experiment sciencesuperschool we will virtually supply  you with the materials, methods and photos of everything you need if you do not have them. We will do the real experiment here in our lab and we will guide you through all the steps, just as though you had everything on hand. Anyone who has an appropriate scale or balance can do this experiment and adult supervision is always required.

Materials needed for the buoyancy experiment:

·        Cork – a cylindrical cork  from a standard wine bottle is great (see photos below)

·        Paper clip or safety pin (for insertion into the top of the cork) is used to  push the cork into the water

·        Small plastic container (such as a medicine vial/bottle)to be filled to the very top (rim) with water and of proper width and depth into which the cork will fit entirely

·        Centimeter-millimeter ruler to measure the dimensions of the cork and any other containers as needed

·        Accurate balance or scale. If a balance or scale is lacking we will provide you with three different measurements and data for that part of the buoyancy experiment.

·        Water for addition to the small vial and into which the cork will be inserted and pushed downward and immersed completely

Buoyancy experiment steps and procedure for corks and related materials

1.      Determine the weight of the cork first. The sciencesupperschool.com cork pictured below weighs 4.2 grams. If you have a balance or scale that can measure grams or ounces measure your cork’s weight and record it in a lab notebook or record, if not use the weight as 4.2 grams.

2.     Measure the diameter of the cork and its height. In this case the sciencesuperschool cork has a diameter of 2 cm (therefore, a radius of 1 cm) and a height of 4 cm.

3.     Determine the volume of the cork (yours or the one used here) according to the formula 3.14(pi) x radius squared x height = volume of cork cylinder. Therefore, our experimental cork volume is:  3.14 x 1 (radius squared =1 x 1 =1) x 4 (height) = 12.56 cubic centimeters (cc). Record the volume of the cork we are using at sciencesuperschool.com as 12.56 cc. This volume of cork should displace 12.56 cc or ml of water and since 1 ml or 1 cc of water weighs 1 gram, then the weight of the water should be 12.56 grams. If you have your own cork, do the proper measurements and calculate the correct volume.

4.     Weigh the water-catching pan or cup to determine that baseline value. The little plastic pan pictured here weighs 2.3 grams.

5.     The pan or cup is placed under the water reservoir vial and is used to catch water that overflows. 

6.     Remember to fill the small water reservoir (plastic or glass container) with water until it is exactly level with the top of the vial. Two cautions: a. Do not spill excess water into the catch pan. B. Do not overfill the water reservoir so that it forms a convex shape at the surface of the tube. This means there is too much water and the excess water and is being sustained by surface tension. The water should be level with the rim.

7.     The buoyancy experiment begins with the water container-reservoir sitting inside the water-catching plastic pan or cup. (See photo below).

8.     Next, insert the paper clip or safety pin carefully into the cork at the top. The safety pin or wire will be used to push the cork into the water reservoir.

9.     Push the cork straight downward until the surface of the cork is level and just covered with water. While doing this note how the water is displaced and is pushed over the sides of the water container. (See photos below). Remove the cork as soon as the water level has just covered the entire top of the cork. This action assures you that the entire cork was immersed in just the right amount of water.

10.  Remove the cork and vial and measure the weight of the water left behind in the collecting pan or cup. Please note and remember in this experiment with the triple-beam balance, I weighed the catch pan or cup alone. This is the measurement obtained: collecting reservoir cup dry = 2.3 grams. Water was added to fill the water reservoir vial and then the whole unit was placed carefully into the collecting reservoir cup. Next the cork was inserted into the water reservoir and pushed down to cover just the the top of the cork and when completed the cork and water reservoir were removed. Weight of the water and the collecting water reservoir pan = 14.9 grams.

11.   To get the actual weight of the water displaced we must subtract the weight of the collecting pan (2.3 grams) from the total weight 14.9 grams to get 12.6 grams of water collected.

12.  The volume of the water displaced was 12.6 cc or 12.6 ml and mass of this water is 12.6 grams. What did we calculate should be the right amount? See above the calculation for the volume of a cylinder and in this case the value is 12.54. The difference between the two values is the actual value is 12.6 – 12.54 (the expected value) = .06 grams. That is truly an amazing result and very accurate.
Why Does a Cork Float in Water and Does This Buoyancy Experiment Prove It?
To keep the buoyancy explanantion simple right now we will simply say that the mass of the cork in this experiment has a downward force of that value of 4.2 grams (times the conversion factor used to convert it to force) and it fully displaces 12.6 grams (times the same conversion factor used to convert it to force). Therefore, this cork causes a upward buoyancy force exactly three times (12.6/4.2) its mass. The cork floats.
Buoyancy Science Experiment Variables to Consider
Why are there differences from experiment to experiment with the same container and the same procedure? Think about that for a minute and come up with some of your own ideas. Human errors occur all the time. Errors such as:

·        overfilling the water reservoir and creating a large convex shape at the top and an excess of water (see photo of surface tension convex water surface below) is one possible error.

·        not filling the water right to the rim would cause a lack of sufficient water.

·        not pushing the cork down so the water just covers the surface

·        pushing the cork too far down

·        not pushing the cork far enough into the water reservoir.  In fact, here are the actual collected amounts of water and the results based on overfilling the water reservoir: 15.9 grams – 2.3 grams = 13.6 grams of water and the results based on underfilling the water reservoir: 13.7 grams – 2.3 grams = 11.4 grams of water. It is truly interesting if you take these two results and average those 13.6 and 11.4 = 25 and divide by 2 you get the 12.5 cc/ml/grams which are very close to the correct and true expected results. These data show us that the more replicas or times we carefully do an experiment, the more likely we are to get the correct final result – that is, the closer you get to the ideal expected answer 12.54.

Another Simple Water Volume and Weight Experiment

The blue glass vial (see photo below) has the following true dimensions: Diameter: 4 cm, height: 16.5 cm. Based on the formula for the volume of a cylinder: (Pi x r squared x height) can you determine how much water by volume (cc or ml) and weight should that container hold? To serve as a starter for your data calculations that blue cylinder pictured below was weighed on the Ohaus Triple Beam Balance  with and without water and the following data were obtained: weight of glass cylinder = 232.1 grams. Weight of the cylinder filled with water = 439.0 grams. How close does the expected volume and weight of water determined by formula and the observed values determined coincide? What is the error as a % of the value obtained? See Solutions link page here for the answer.

 

Buoyancy or the upward force of hot air in this balloon causes it to rise as the heated air enters the balloon.When the balloon is rising it is displacing more surrounding air mass than the whole mass of the balloon. The upward force of buoyancy is greater than the downward forces of air and gravity. If the air cools, the balloon will descend. The balloon begins to descend when the mass of air it displaces is less than the total mass of the balloon. This means that the downward force of air and mass x gravity (weight) is greater than the upward force of the balloon. If the air in the balloon slowly escapes or the trapped air is cooled the balloon begins to descend. If he ballon pilot heats the air with flames the warm air expands the balloon which begins to displace more surrounding air and the balloon rises. If all the air leaves the balloon, it crashes to the ground. The arrows indicate the sideward, upward and downward pressures or forces on the balloon. Photo Credit: U.S. Centennial of Flight Commission
 
 
Photo: Ohaus Triple Beam Balance Adjusted to Zero. Weigh Pan on Left, Pointer to far right is cnetered. Balance is ready to be used for weighing.
Photo Copyright: Donald Reinhardt
 
Bouyancy Experiment Materials: Water Reservoir, Water- Catch Cup and Cork with Safety Pin Inserted
Photo Copyright: Donald Reinhardt, c. 2012
 
Bouyancy experiment with cork immersion into water reservoir with water spill over into plastic water-catch cup.
Photo Copyright: Donald Reinhardt, c. 2012 
 
Buoyancy experiment with water-catch cup and water spillover set on Ohaus weight pan for determining mass-weight of water.
Photo Copyright: Donald Reinhardt, c. 2012 
 
Buoyancy experiment with a blue glass cylinder used to determine volume of cylinder and total volume and weight of water. See article above on Buoyancy to see experiment, materials and methods, and the results.
Copyrighted Photo: Donald Reinhardt, c. 2012
See more experiments and ideas on buoyancy here: