Most people will not see the results of this little demonstration coming, especially if it's done as an immediate follow-up to the previous project. You can make a can actually roll uphill for a short distance without violating any of Newton's laws.
What You Need
- coffee can
- weight—an old battery should work well
- strong glue—such as Gorilla glue or epoxy
- Glue the weight to the inside of the coffee can.
- Conceal the interior with the plastic cover of the coffee can.
- Place the can on the incline with the mass on the uphill side of the can's centerline. See Figure 62-1.
- Ask someone observing this what they think will happen. You can add to the effect by creating the illusion that you are investigating one of the situations of the previous discovery by using two coffee cans with weights.
Once released, the can rolls uphill a few inches.
Why It Works
Gravity pulls on the weight, which exerts a torque on the can. If the weight is positioned on the uphill side of the can's centerline, the can will roll uphill (until the weight is brought directly under the can's center of gravity).
Other Things to Try
Another similar idea is to attach a rubber band with a weight in the middle to the inside of the can. As the can rolls, the rubber band is wound up. When the can rolls, it reaches a turning point, and then returns back in the direction from which it came as the rubber band with the weight unwinds.
If the force exerted by gravity on an object's center-of-mass produces a torque on the object, the object can briefly roll uphill.