Key Concepts
Physics
Mass
Potential energy
Kinetic energy

Introduction
It is fun to race, especially when the competition is evenly matched! In this race the force of gravity provides the power and the competitors are the same size, weight and distance from the finish line. Gather the materials and see who the winner will be!

Background
If you roll two soccer balls down a hill, they should roll similarly—and take about the same amount of time to complete the path. But what if you had two objects that were the same size and the same weight—but had their weight distributed differently? Two objects like this may behave differently when they roll downhill. How quickly an object accelerates depends partly on how its mass is distributed. How? Gather some basic materials and set up a race!

Materials

• Two identical round metal cookie tins (the type butter cookies come in)
• 10 large metal washers, about one quarter pound each
• Double-sided foam tape
• Sidewalk chalk
• Hilly street with a smooth, clear sidewalk

Preparation
• Using double-sided foam tape as an adhesive, arrange five of the washers evenly around the rim of the bottom of one metal cookie tin.
• Use double-sided foam tape again to secure the other five washers to the middle of the bottom of the second container.
• Gather all materials and head outside. Find a hill with a smooth sidewalk clear of cars, pedestrians and obstacles.

Procedure
• Use chalk to mark a start line at the top of the hill.
• Use chalk to mark a finish line at the bottom of the hill.
• Place both tins at the starting point.
• Do the tins feel the same in your hand? Which tin do you think will reach the finish line first? Why?
• Release both tins at the same time so that they can race down the hill. What do their rolling patterns look like?
• Did they reach the finish line at the same time? Was one tin faster than the other?
• Try racing the tins down the hill again. Did you get the same results?
• Extra: It is also interesting to experiment with rolling cans of soup down an incline. Get a can of thin broth soup and a can of thick cream soup and roll them down the hill. Did one type of soup consistently win? Why might this be?

Observations and results
The results of the race have to do with potential versus kinetic energy. At the start of the race, both cookie tins have identical potential energy because each has the same mass and the same dimensions. But because the washers are taped in different places (the outside of one tin versus the center of the other) each tin has its mass distributed differently. This affects how quickly the object can accelerate.

The tin with its mass at the rim will use a greater part of its original potential energy just to get rolling. The tin with its mass concentrated at the center has an easier time to get rolling and thereby doesn't use as much of its potential energy. This means the tin with its mass at the rim has less energy available to translate into kinetic energy, resulting in a lower linear speed.

If you tried racing the cans of soup, a similar difference in kinetic energy occurs. The thin, liquid broth stays relatively in place and does not rotate when the can does. This means the potential energy of the broth can go into linear motion, not into rotating the can. The thick, creamy soup is like a solid inside of the can and rotates with it. This means some of the potential energy of the thick soup is needed for rotational energy; this makes the can roll slower.

More to Explore
Mechanics and Motion, from Physics4Kids.com
Rotational Motion (pdf), from Springboro.org
Downhill Racer, from Exploratorium
Snappy Science: Stretched Rubber Bands Are Loaded with Potential Energy!, from Scientific American

This activity brought to you in partnership with Exploratorium