Name ____________________________________________ Hour ___

Archemide's Principle of Buoyancy
http://home.a-city.de/walter.fendt/physengl/buoyforce.htm

This Java applet shows a simple experiment concerning the buoyancy in a liquid:A solid body hanging from a spring balance is dipped into a liquid (by dragging the mouse!).

In this case the measured force, which is equal to the difference of weight and buoyant force, is reduced.

You can change (within certain limits) the pre-selected values of base area, height and densities by using the appropriate text fields.

After you have pressed the "Enter" key, the program will indicate the new values of:

drought or the depth of the object when placed in the liquid.

replaced volume or volume of the liquid displaced by the object when submerged.

buoyant force or the upward force of liquid on the object.

weight or the force of gravity on object when in air.

measured force or the force which is equal to the difference of weight and buoyant force (usually this value is reduced when object is placed in the liquid.)

A gravitational acceleration of g = 9.81 m/s2 was presupposed.

If you see the words "Maximum exceeded!" (red letters), you have to choose an adequate measuring range.

Law of Archimedes:

The buoyant force is equal to the weight of the replaced liquid or gas.

Procedure:

Set the program to these values: Measuring Range = 50 N,

Base Area = 40 cm3, Height of body = 10 cm,       

Density of body = 2 g/cm3, Density of liquid = 4.0 g/cm3

Use the web site program to complete the table below and

Calculate the following values and record them in the table:

Volume of the body = Base Area x Height of the body

Base Area of body	Height of the body	Density of the body	Volume of the body	Object floats or sinks?	Density of the liquid	Draught	Displaced Volume of Liquid	Buoyant Force	Weight of body	Measured Force	Density Ratio	Volume Ratio
								(up)	(down)	(net F)
cm2	cm	g/cm3	cm3		g/cm3	cm	cm3	N	N	N
50	10	1			5
40	10	2			5
40	10	3			5
40	10	4			5
40	10	5			5
40	10	6			5
40	10	8			5
40	10	10			5

Calculations:

• Calculate the ratio of the object density to the liquid density. Record in chart as "density ratio".
• Calculate the volume of the object to the volume of displaced liquid. Record in chart as the "volume ratio"

Questions:

1. How do the density ratio and volume ratio compare in the chart above? _________________

2. When an object floats, which has greater density the liquid or the object? ________________

3. When the object floats, how does the buoyant force compare to the weight of the object? _________________________________________

4. When the object sinks, how does the buoyant force compare to the weight of the object? _________________________________________

5. When is an object "weightless" (measured force = 0.0 N) when placed in a liquid? ______________________________________________________________________

6. Is the buoyant force equal to the displaced volume of liquid? Use the formula and data below to prove it. Use data from one row in table above.

• Mass = Density x Volume:

(Displaced liquid volume in cm³) x (density of liquid g/cm³) = (mass of liquid displaced in gram)

• Convert mass in grams to kilograms:

Mass of displaced liquid divided by 1000 kg/g = _____________ mass in kilograms.

• Convert mass to the force known as weight:

Mass of displaced liquid in Kg x 9.8 m/s² = __________________weight in newtons.

• Compare the two forces, weight and buoyant force:

Buoyant force = _____________ newtons.

How do the last two answers compare? ____________________________________

Do you agree with Archemede’s Principle that the buoyant force is equal to the weight of the displaced liquid? ___________________