### Can Races

Which wins? A marble and a greased box are both started at the same point on an incline. The marble rolls while the greased box slides down the incline.

The box wins every time!

What would happen if an unopened soup can were used instead of the marble?

Introduction

A body rotating about its axis has rotational kinetic energy. If it is rolling across a surface, it also has translational kinetic energy. Part of the body's total energy is rotational and part of it is translational. In our example, part of the marble's gravitational potential enerery (GPE) was converted to rotational kinetic energy, leaving less for the translational kinetic energy.

When several different objects are rolled down the incline, the translational speed at the bottom of the incline depends upon the moment of inertia of the object. The moment of inertia expresses how the mass is distributed. An object represented by a hoop will have the lowest speed and arrive at the bottom of the incline last. It has most of its mass concentrated at its radius. For all objects, the speed at the bottom of the incline does not depend upon the object's mass, but only on its shape.

If no heat enters or leaves AND if no work is done on or by the system, energy just changes from one form to another.

Objective

• Students will predict which can they think will win the Can Race.
• Students will predict why they think which can will win the Can Race.

Materials

• long ramp. A piece of plywood is best, as several cans can be "raced' simultaneously, allowing easy comparison.
• support for ramp, raising its upper end approximately 50 cm
• student-provided cans (do not allow soft-drink cans)

Procedure

1. Collect all can "entries." Have students predict which can they think will win.
2. Have students measure the radius of their cans.
3. Put cans at the top of the plywood, holding them at the same height.
4. Release the cans.
5. If there are too many cans to run at one time, "race" the top three finishers of each qualifying round in a "championship heat."

Questions

1. Which can did you predict would win the can races?
2. Describe the can. Describe its contents. What was its radius? How is the mass of the can and its contents distributed?
3. Why did you think that this can would win?
4. Which can won the can races?
5. Describe the can. Describe its contents. What was its radius? How is the mass of the can and its contents distributed?
6. Why do you think that this can won?

Project Evaluation

• Each question is worth 10 pts.
• You will receive 40 points for providing a can. These cans of food will be donated to a community food bank after the project's completion.

Extension

Roll a coffee can filled with water down the incline. Roll a coffee can filled with a gell agent down the incline.