ElizabethE:Buoyant Force

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What do you think your students should know about buoyant force?

1) The fluid exerts force on all sides of a submerged object.

2) The net force on an object (Buoyant force) is up and why.

3) The bouyant force depends on the density of the fluid and the volume of the fluid displaced, not on the density of the object.

4)$Fb=pgV$

How will they learn it?

1) The fluid exerts force on all sides of a submerged object.

Students will do this during the mathimatical derivation of the bouyant force discussed below. Have students draw pressure diagrams for an object submerged in a fluid. Students will then use the relationship between force and pressure ($P=F/A$) to draw a free body diagram.

2) The net force on an object(Buoyant force)is up and why.

Students will learn this by first doing qualitative observational experiments. The basic idea of this experiment is to see how the reading on a spring scale changes as an object is lowed into a fluid and stops changing when the object is fully submerged.

3) The bouyant force depends on the density of the fluid and the volume of the fluid displaced, not on the density of the object.

Next student will do a qualitative observational experiment in which they mesure the change in scale reading and displaced volume (Students may also be provided with a data table instead.). The students will analize the data to determing that the change in volume is directly proportional to the change in scale reading (i.e. $Fnet=C*deltaV$ where C is a constant). Finaly stdents will repeat the experiment with a fluid of differnt density and analize why C is different. Students will determin that $C=pg$.

4)$Fb=pgV$

Here the students will do a mathimatical derication starting with the pressure on a submerged object or volume of fluid. Students will need to draw force and pressure diagrams. Students will determine the pressure on all sides $P = Patm +pgh$ and then use the equation that relates force to pressure $P=F/A$ to find the force on all sides. Finally they will use Newton's second law to find the net force $Fnet=(Patm+Pbelow)*A-(Patm+Pabove)*A=(pgh2-pgh1)*A=pgV$. This is the bouyant force.

5) Testing Experiment

Students will test Arcamedies Principle. For this you could use a third fluid of know density and ask students to predict the scale reading when a certain amout of the fluid is displeced. Archimedies buckit could also be used.

How will you assess them? (Make sure that your assessment addresses typical student difficulties)

To assess students knowlege you may ask them tp explain certin phinominon such as why a raw egg or patato floats in one fluid, but not in another. You might also ask about a coke and diet coke can in water. Why does one float when the other does not?

The cartisian diver may also be used as an assessment. However this is more difficult since the students must also consider the change in mass of the idrober that occours when the pressure increases.

Taken from "Helping students develop an understanding of Archimedes' principle. I. Research on student understanding" by Loverude, Kautz and Heron Department of Physics, University of Wachington, Seattle, Washington

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