Try this: Uphill roller

I recently found a video clip of a car commercial on a web site, which shows a long series of things knocking into each other. It's an impressive advert, especially since the makers did not use any computer effects.

There is one part that looks like it should be impossible. At one point in the advertisement, three wheels roll uphill, one after the other. This week, we will see how it happened.

You will need

• A light, cylindrical container. A tin can or a plastic storage jar will work well. The wider it is, the better it will work.
• Several rubber bands.
• Plasticine. You need a lump that is heavier than the container.
• A ramp where you can adjust the steepness. I just used a flat tray and a pile of books to make mine.

What to do

1. Wrap some rubber bands around the container. This will give it some grip when it rolls along.
2. Roll the plasticine into a cylinder, about as long as the container is high.
3. Stick the plasticine onto the inside of the container, along one side of the container.
4. Place container on its side on the ramp, with the plasticine at the top.
5. Roll the container uphill a short distance (less than a quarter of a turn of the container) without letting go of it.
6. Let go of the container. You should find it will roll up the ramp, until the plasticine is at the bottom of the container.

You may need to adjust the angle of the ramp and the starting position of the plasticine in the container to make this one work. Once you have the right combination, it should be easy to make your container roll uphill.

What's happening

Every object has a point called the centre of mass, which is sometimes called the centre of balance or centre of gravity. An object's centre of mass is the point it would balance on. Gravity pulls down on an object as though all of that object's mass were concentrated at that one spot.

When the container is empty, its centre of mass is at the centre of the container. If you place an empty container on the ramp, it feels the force of gravity down through its centre of mass, so it is pulled down the ramp.

Since the plasticine is heavier than the container, their combined centre of mass is close to the plasticine. When you put the weighted container on the ramp, the centre of mass was near the top. If the slope of the ramp is gentle enough, then as the container rolls uphill, its centre of mass will actually move downwards (there is a diagram of this on the web site). For it to roll down, the hill, it would actually have to lift its centre of mass upwards, against the force of gravity.

Although the can rolls uphill, the centre of mass (marked in blue) moves downwards.

Applications

In the car commercial, three ordinary-looking wheels rolled uphill. In fact, one side of each wheel was packed with nuts and bolts from the car, so that when the wheels rolled uphill, their centre of mass was moving downwards, just like our container.