Materials
- A ramp about one meter long
- A can of cream of broccoli soup (98% fat free if able)
- A can of beef broth
- Stopwatch
- Meter Stick
Procedure
- If you look in the store you can usually find a broth type soup and a cream type soup in the grocery store whose cans are the same size and whose mass is also the same. This works the best for this experiment.
- Suggestion to have on the front board when walking in:Ladies and Gentlemen, you are about to witness a pivotal event. The annual Great Soup Can Race!Please allow me to introduce our contestants:
The reigning champion, Beef Broth, weighing at a spectacular 12.5 fluid ounces has won the competition numerous times and is confident of demolishing the competition in our race today.
Introducing also, Cream of Broccoli soup, she’s meaner and leaner. Now appearing 98% fat free and also weighing in at 12.5 fluid ounces. Cream of Broccoli has been training all year and is convinced that this is the year she will take home the championship. Who do you think will win?
(Write who you think will reach the bottom of the ramp first if both soup cans are released at the same time and allowed to roll down the ramp on their sides. Be sure to include the reason you think the way you do.)
- Have the students predict which can, if rolled down an incline, will reach the bottom of the hill first.
- Have the students start both of the soup cans rolling at the same time.
- A meter stick can be used to stop the cans from rolling originally and to release the cans at the same time.
- Let the cans roll down the incline. Beef Broth will win every time.
- This activity can be extended by having students time each can and calculate the amount of potential energy at the top of the hill compared to the kinetic energy at the bottom of the hill. The students will find in both cases that there is some energy lost, this energy is mostly rotational energy.
Explanation
Cream of Broccoli soup is more of a thick paste, so when this can is rolling down the ramp, the soup rotates along with the can as one solid object, like a solid cylinder. The Beef Broth is more of a liquid, so the can rolls, while the liquid stays oriented up and down, therefore the can rolls like a hollow cylinder. The rotational inertia of a solid cylinder is much greater than that of a hollow cylinder. Imagine trying to spin a 10 lb and a 15 lb bowling ball. It takes more energy to get the 15 lb bowling ball to rotate at the same rate as a 10 lb bowling ball.
Even though both soup cans have the same about of energy at the top of the ramp, the energy gets divided up differently as the soup cans roll down the ramp. The potential energy of the can at the top of the ramp gets converted into kinetic energy and rotational energy. Since it takes more energy to rotate the Cream of Broccoli soup, less energy is used for kinetic energy and the Cream of Broccoli soup rolls slower.
Reinforcement Activities
Have students feel the effects of angular momentum by using a gyroscope, once the gyroscope is spinning it will automatically right itself. Have the students hold the gyroscope in their hands and try to rotate the gyroscope to feel the force acting on it.
For an even more impressive demonstration, have the students use a bicycle wheel as a gyroscope. They can even use the gyroscope to spin themselves in a rotating chair. This shows the principle of conservation of angular momentum.
Have a contest of which student can create the slowest rolling spool down a ramp. Have students use what they learned about rotational inertia to their advantage and have fun doing it.
