PH202Winter 2002

Laboratory #1 : Archimede’s Principle

The purpose of this lab is to investigate the principle of buoyancy, to measure the apparent weight of objects and to apply Archimede’s principle to determine the density of a solution.

Archimede made his discovery without the benefit of Newton’s insights. Since you have Newton’s achievements as part of your intellectual heritage ( PH 201) we’ll use the concepts of force and equilibrium to measure the buoyant force acting on a partially or totally submerged object.

EXPERIMENT

I) Hang an object from a force sensor in the configuration shown in the figure bellow. The object could be partially or totally submerged.

Force sensor FFFO

fluid

According to the Archimede’s Principle, the upward buoyant force B which the liquid exerts on the object, is equal to the weight of the fluid displaced, or:

V= Volume of the fluid

(1)B= V gdisplaced

=density of the fluid

The force sensor will measure the net hanging force, which should be equal to the weight of the object minus the buoyant force B:

(2)F= mg-B

Combining eq. (1) and (2) we obtain

(3)F=mg-(Vg)

Procedure: Set up the experiment and measure the net hanging force on an object as it is lowered into a graduated cylinder. Plot the measured buoyant force versus the volume of the water displaced. The slope of the linear fit should be equal to ( )(fill in the blank). Use the slope of the graph to determine the density of the water.

=______% discrepancy=______

II) a) Put the graduated cylinder with water on a digital scale. Record the reading. Suppose that, by the means of a string, you submerge an object into the water (partially). Do you expect the indication of the digital scale to change? Why? Write your prediction and explanation bellow.

______

b) Now submerge an object partially into the water. Read the indication of the scale and the volume displaced. Plot the B versus V graph and determine the density of the water.

=______%discrepancy=______

III) a) Measure the mass of 2 teaspoons of salt, then stir it into the water in the graduated cylinder. Repeat the experiment I or II and use the slope of the new graph to determine the density of the solution.

M salt measured=______ solution=______

b)Using the density of the solution and the volume of the solution, estimate the mass of salt added. Show your calculations bellow.

M salt calculated=______%discrepancy=______

IV) a)A coin made out of an alloy of silver and copper can be weighted in air and then in water (apparent weight). The difference between the weight and the apparent weight is the buoyant force. Show (theoretically) that, using only these 2 data, you can calculate how much Cu is there in the coin.

b) Determine the ratio Cu to Ag in a bracelet by measuring the weight and the apparent weight in water.