Fish Physics

Alverno Science Inquiry Activities on the Internet

Activity: Make and test hypotheses about hydrodynamics and bouyancy
Level: High school, beginning college
Time: 1.5 hours (can be split into two days)
Equipment needed for each pair of students:

Equipment needed for class to share: Objectives
After successfully completing this activity, the student will be able to:


The hydrodynamics activity allows students to make and test hypotheses about how the shape of a fish affects its movement through water. Gravity is used as a substitute for fish muscle power to provide motion. The bouyancy activity allows students to experiment with different objects and observe which sink, which float, and how bouyancy can be altered without changing the mass of an object. Students are also asked to relate their findings from the activities to the anatomy of fish.

Author: Rebecca Burton, feel free to email questions and comments from you and your students.Creation and maintenance of this web site was made possible by NSF-ILI Grant DUE 9750658.


Fill tanks or jars with water. For the hydrodynamics activity, have students weigh out their clay, then follow the directions in the worksheet below. You might have a quick fish race tournament with the class winner challenging the instructor's fish. For the bouyancy activity, provide balloons of different bouyancy in a demonstration tank, or make enough for each pair of students to take them back to their own tank. Vary the air, water, and beads in the balloons to make sinkers, floaters, and balloons with no air that will sink if the beads are pushed together, but float if they are spread out and the balloon is placed flat on the surface. Then have students follow the directions in the worksheet below.


What made some fish faster than others?
What affect did adding fins or a tail have?
Why aren't all fish shaped the same way?
What makes objects float?
How can boats be made of materials that are denser than water?
What is the advantage of having a swim bladder?
Why don't all fish have them?

The following worksheet can be cut and pasted. If you use it, or any part of it, please include the source information.


1. Mold your clay into a pancake shape. Rest it on the top of the water. Let go and time its descent. Repeat this 2 more times and get the average time.

2. Mold your clay into a cube or sphere. Do the same procedure. Write your mean time.

3. Mold your clay into the shape of a fish. Repeat the procedure (pointing the "nose" of the fish down) to get your mean time.

Which was faster? Why?

4. Make some hypotheses about modifications that will increase the speed of your fish. Materials are available for adding "fins".

5. Shape your "fish" and time it to test your hypotheses. Remember to test one modification at a time.

6. Using your fastest fish design, challenge another team to a fish race. Do at least 3 trials.

7. Draw the shape of the winning "fish"

8. Why was this a good shape?

9. If some shapes are faster than others, why aren't all real fish shaped alike?


1. Take a look at the various balloons in the tank. Feel free to handle them.

2. Can you make any of the sinkers float or floaters sink? If so, describe how.

3. Push two floaters to the bottom. Do they rise at the same rate? Why?

4. Pull two sinkers to the top. Do they sink at the same rate? Why?

5. Why do some balloons sink and some float? List some hypotheses.

6. Experiment with your own balloons, adding or removing weights, air, and water to test your hypotheses.

7. Which of your hypotheses were supported?

8. How do your findings relate to the swim bladder in some fish?

9. What happens if you put oil in one balloon and an equal amount of water in another balloon? How might this phenomenon affect sharks, which have a big, oily liver?

10. When would a fish benefit by being a sinker?

11. When would a fish benefit by being a floater?

12. When would a fish benefit by having a neutral buoyancy?

Developed by Rebecca Burton, Alverno College Biology Dept.

Return to Alverno Activities on the internet
Last update: 4/18/02 by Rebecca Burton, Dept. of Biology, Alverno College