MnSTEP Teaching Activity Collection > MnSTEP Activities > Buoyant Force

# Buoyant Force

#### Summary

In this Physics lab, students learn about Archimedes' Principle. An object is submerged in water and vegetable oil and the buoyant force and height are measured at different levels. The slope of the graph Buoyant Force vs. Height is the proportional to the density of the fluid. Student will write a lab report describing their results.

## Learning Goals

Students will be able to:
1. Use the Force Sensor to measure the buoyant force acting on a submerged object.
2. Use Keyboard Sampling to enter measured depth values
3. Use the Graph display to determine the slope of a force versus depth plot.
4. Apply Archimedes' Principle to determine the fluid density from the slope of the graph.
5. Compare a calculated value for density with an accepted value.

## Context for Use

- 9-12 Grade Level
- Lab
- 60 minutes needed for this activity, this includes setup.
- Data Studio with Scientific Pasco equipment is needed.
- Students should have been introduced to buoyant force before beginning this lab.

Subject: Physics:Fluid Mechanics
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)

## Description and Teaching Materials

Introduction: Before beginning the lab have the students answer some "Thinking Questions", like:
1. Why is it easy to float in the ocean?
2. In which would you feel a stronger buoyant force: a swimming pool filled with oil or with syrup?

Background: Archimedes' Principle states that the buoyant upward force on an object entirely or partially submerged in a fluids is equal to the weight of the fluid displaced by the object. If the object if lowered into the fluid while the buoyant force is measured, the slope of the graph of Buoyant Force vs. Height is proportional to the density of the fluid.

Materials Needed:
1. Economy Force Sensor
2. Base and Support Rod
3. Beaker, 1000ml
4. Calipers
5. Clamp, right-angle
6. Cylinder, with hook
8. Lab Jack
9. Mass and Hanger Set
10. Ruler, metric
11. String
12. Support Rod
13. Water
14. Vegetable Oil

Students Do:
1.Use the Force Sensor to measure the force on an object as it is lowered into water.
2. Use 'Keyboard Sampling' to enter the depth values.
3. Use DataStudio to plot the force versus submerged depth to obtain the density of the fluid.

Need to include in lab report:
1. "Think" questions and answers from introduction
2. Data Table
- Area of Cylinder
- Slope(from graph)
- Density of water (calculated)
- Density of water (accepted)
- Density of vegetable oil (calculated)
- Density of vegetable oil (accepted)

- How does your experimental value compare to the accepted value for the density of water and oil? What is the percent difference?
- Why was the Force Sensor zeroed after the cylinder was attached to the hook?
- Is vegetable oil less, or, or equally as dense as water?

## Teaching Notes and Tips

Teachers Notes:
1. A 2-cm diameter by 6-cm long cylinder is recommended.
2. The kind of material used for the cylinder does not matter as long as the material does not float in water or in vegetable oil.
3. The best-shaped object to use is a cylinder with a flat bottom and top, and vertical side.
4. Have students tare the Force Sensor after the cylinder is attached to compensate for the weight of the cylinder because the Force Sensor measures the net force (cylinder's weight minus the buoyant force).

## Assessment

I use two students to a lab station and spend a lot of time at each station listening to students and asking questions as they work.

Students are required to do a lab report for each lab.

These two assessments give me a good sense of student understanding.