Jump to main content

Buoyancy of Floating Cylinders

1
2
3
4
5
135 reviews

Abstract

This science project presents an interesting puzzle. A disk of wood will float face-up; that is, with its circular cross-section parallel to the surface of the water. A long log of wood, however, floats on its side with the circular cross-section perpendicular to the surface of the water. If you think about it, disks and logs are both cylinders. Is there some intermediate length of cylinder that floats with the circular cross-section at a tilted angle? Try this experiment to find out!

Summary

Areas of Science
Difficulty
 
Time Required
Short (2-5 days)
Prerequisites
None
Material Availability
Readily available
Cost
Low ($20 - $50)
Safety
Adult supervision is recommended when using the wood saw. Always wear safety goggles when working with tools.
Credits
Andrew Olson, PhD, Science Buddies
Justin Spahn, Science Buddies
Edited by Sandra Slutz, PhD, Science Buddies

Sources

This science project was suggested to Science Buddies by Stewart Levin, PhD, who got the idea from the following article:
  • Gilbert, E.N. (1991). "How Things Float," The American Mathematical Monthly, 98 (3, March): 201–216.

Objective

The goal of this science project is to measure how the tilt angle of cylinders floating in water depends on the aspect ratio (length/diameter) of the cylinder.

Introduction

If you place a wooden disk in water, it floats 'face up,' i.e., with the circular cross-section parallel to the surface of the water. However, if you place a long wooden cylinder in water, it floats on its side with the circular cross-section perpendicular to the surface of the water (see Figure 1).

Drawing of a disk and cylinder floating an a container of water
Figure 1. Illustration of a floating disk (A) and a floating cylinder (B).

If you think about it, a disk is a cylinder, too. A disk is just a very short cylinder, and disk is just a special name for this type of cylinder. How short does a cylinder need to be before you can call it a disk, or is there something more to it? A coaster for a hot cup of coffee certainly fits our concept of a disk. A ceramic coaster might be almost 1 centimeter (cm) tall and 10 cm in diameter. However, you wouldn't call a 1-cm length of pencil lead a disk, you'd call it a cylinder. That's because the diameter of the pencil lead is only 0.05 cm (0.5 mm). So apparently, you should consider both the length and the diameter of a cylinder when deciding whether or not it's a disk.

A handy way to consider both numbers at once is to use a ratio. For example, if you use the ratio:

Equation for the aspect ratio of a cylinder is the cylinder's length divided by its diameter

The coaster has an aspect ratio of 1/10, and the pencil lead has an aspect ratio of 1/0.05 or 20. So perhaps what disk means is a cylinder with an aspect ratio < 1.

Does the way a cylinder floats also depend on its aspect ratio? Since the disk floats face-up, but a longer cylinder floats on its side with the circular faces perpendicular to the surface, does that mean that there are cylinders with intermediate aspect ratios that would float at intermediate angles? Try this science project to find out!

Terms and Concepts

Questions

Bibliography

Materials and Equipment

Experimental Procedure

  1. With an adult's help, use a hand saw to cut cylinders of various lengths from a long piece of dowel. You'll need to experiment and figure out what range of lengths you need in order to see different tilt angles in water.
  2. Measuring the aspect ratio of your cylinders is easy. Just measure the length (in cm) and the diameter (in cm), then divide the length by the diameter.
  3. Prepare the gelatin-and-cylinder setup, as follows:
    1. Using the package instructions, pot, measuring cup, and stove, make 2 qt. of colored gelatin.
      1. You'll need two 6-ounce (oz.) packages to make 2 qt. of gelatin. This works best if you use a strong color like blue, red, or purple, If you have clear gelatin, you can add your own food coloring.
      2. Use caution around the hot liquid gelatin.
    2. With an adult's help, pour the liquid gelatin into the dish pan.
    3. Carefully place the dish pan filled with gelatin in the refrigerator to set. Then gently place the dowels in the liquid, as far away from the edges as possible, making sure that they don't roll too much. Note: Rolling a little isn't a problem as long as the dowels finally come to a rest so that they are not touching either the sides of the container or another dowel.
      1. You might need to occasionally nudge the cylinders away from the edge of the dish.
    4. Once the gelatin has set, approximately 4 hours later, remove the cylinders from the gelatin.
    5. The food coloring in the gelatin will stain the submerged portion of each cylinder. There will be a distinct line of dye making the water line on each cylinder.
  4. Use the following steps to measure the tilt angle of each cylinder:
    1. Using a pencil and a ruler, draw a straight line on a piece of paper.
    2. Place the dyed cylinder over the straight line, and tilt it until the dye line on the cylinder is parallel with the line on the paper (Figure 2A).
    3. Holding the cylinder in place, place a ruler against the cylinder at the same angle. (Figure 2A).
    4. Move the cylinder out of the way and use the ruler to draw a straight line that intersects with the original line on the paper.
    5. Use your protractor to measure the angle between the two lines (Figure 2B).
    6. To keep track of your measurements, we suggest that you use a separate sheet of paper for each cylinder. Label each angle drawing with the length, diameter, and aspect ratio of the cylinder.

      Drawing of a protractor measuring the angle of the water line left on a floating cylinder
      Figure 2. Measuring the tilt angle of the dyed dowel.

  5. Make a data table of your results in your lab notebook:


    Length
    (cm)
    Diameter
    (cm)
    Aspect Ratio
    (length/diameter)
    Tilt Angle
    (°)
           
           


  6. Make a graph of your results by plotting tilt angle (y-axis) vs. aspect ratio (x-axis). Over what range of aspect ratios does the tilt angle change?
  7. Repeat steps 3–6 of the procedure twice more using the same cylinders. Repeating the procedure allows you to determine if your observations are reliable.
    1. Do the graphs look similar for each repeat of the experiment?
    2. Over what range of aspect ratios does the tilt angle change?
icon scientific method

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Global Connections

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

This project explores topics key to Industry, Innovation and Infrastructure: Build resilient infrastructure, promote sustainable industrialization and foster innovation.

Variations

  • Try dowels of different diameters, but with the same density. For each diameter, cut dowels of various lengths and measure their flotation angles. Make a graph of flotation angle (y-axis) vs. length of the cylinder (x-axis). Use a distinct symbol for each diameter. How do the graphs compare for each cylinder diameter? Now make a graph of flotation angle (y-axis) vs. aspect ratio of the cylinder (length/diameter) (x-axis). Use the same symbols as before. How do these graphs compare?
  • Try cylinders with different densities. Is the relationship between flotation angle and aspect ratio the same or different? Can you find cylinders made of different materials but with the same density? Do they have the same relationship between flotation angle and aspect ratio?
  • For more advanced students: Can you come up with an explanation of the physics behind the tilt angle vs. aspect ratio relationship? Can you figure out an equation that describes the relationship between tilt angle and aspect ratio? The following article will be a useful reference: Gilbert, E.N. (1991). "How Things Float." The American Mathematical Monthly, 98 (3, March): 201–216.

Careers

If you like this project, you might enjoy exploring these related careers:

Career Profile
Water covers more than 70 percent of Earth's surface, and marine architects design vessels that allow humans and their cargo to cross through or under those waters safely and efficiently. Some of their watercraft designs are enormous, like merchant ships, which carry huge loads of oil, cars, food, clothing, toys, and other goods, across thousands of miles of open waters. These ships are essential for trade between countries. Other vessels are smaller and more specialized, like luxury yachts or… Read more
Career Profile
Mathematicians are part of an ancient tradition of searching for patterns, conjecturing, and figuring out truths based on rigorous deduction. Some mathematicians focus on purely theoretical problems, with no obvious or immediate applications, except to advance our understanding of mathematics, while others focus on applied mathematics, where they try to solve problems in economics, business, science, physics, or engineering. Read more
Career Profile
Our universe is full of matter and energy, and how that matter and energy moves and interacts in space and time is the subject of physics. Physics teachers spend their days showing and explaining the marvels of physics, which underlies all the other science subjects, including biology, chemistry, Earth and space science. Their work serves to develop the next generation of scientists and engineers, including all healthcare professionals. They also help all students better understand their… Read more
Career Profile
Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to… Read more

News Feed on This Topic

 
, ,

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Buoyancy of Floating Cylinders." Science Buddies, 20 Nov. 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Aero_p021/aerodynamics-hydrodynamics/buoyancy-of-floating-cylinders. Accessed 19 Mar. 2024.

APA Style

Science Buddies Staff. (2020, November 20). Buoyancy of Floating Cylinders. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Aero_p021/aerodynamics-hydrodynamics/buoyancy-of-floating-cylinders


Last edit date: 2020-11-20
Top
We use cookies and those of third party providers to deliver the best possible web experience and to compile statistics.
By continuing and using the site, including the landing page, you agree to our Privacy Policy and Terms of Use.
OK, got it
Free science fair projects.