Science -- Your Future, Scotland's Future
SCI-FUN Roadshow Exhibits -- Rattleback
SCI-FUN Roadshow Exhibits -- Rattleback
In this exhibit you learn about how asymmetry can affect the movement of things.

Things that are symmetrical move the same way in both directions. The Rattleback, however, is not symmetrical, so turns anticlockwise, but not clockwise.

This is because the curve on the bottom of the Rattleback is not in line with the length of the Rattleback; it is squint. This means that there is more friction when the Rattleback turns clockwise, and turn is reversed.

Asymmetry can affect the movements of many different objects and animals. Imagine that you have one leg shorter than the other. If you move both legs at the same speed, the shorter leg will make smaller steps than the longer leg, and this will cause you to walk in a curve in the direction of the shorter leg. In order to walk in a straight line, you must take bigger, faster steps with the shorter leg.

Because of this asymmetry is rare in both the animal kingdom and vehicles. Most animals are externally symmetrical, which allows them to move easily in a straight line and turn in both directions (although some fictional animals have one leg shorter than the other to run round hills, including the Sidehill Gouger, Wild Haggis and Dahu). A few animals show marked asymmetry, but this is often to give them a weapon of some kind, such as the claw of the fiddler crab, or the tusk of the narwhal. Owls have wonky ears, which allows them to better locate where sound is coming from. Flatfish are extremely asymmetrical, and actually lie with one side on the ground and one side facing up; these two sides are very different to each other in colouring and both eyes are on one side of the fish.

Engineers tend to make vehicles externally symmetrical. Although a car has a driver's side and a passenger's side on the inside, the outside is the same on both sides. The wheels are the same on both sides, and the chassis is the same on both sides. It is important that friction and air resistance is the same on both sides of the vehicle, and that propulsion is the also the same on both sides. Boats and aeroplanes are also always made to be symmetrical on the outside.

1 Things that are symmetrical move the ________ way in ________ directions.
2 In which direction would you turn if your left leg were shorter than your right?
3 Why are most animals symmetrical?
4 What useful things might some animals gain by being asymmetrical?
5 What forces must be the same on either side of a car?
6 How do you think a boat would move if its hull were the same shape as the Rattleback?

1 Can you make asymmetrical toy cars? Get two toy cars with different sized wheels and swap the wheels on one side of one car with the wheels on one side of the other. Now push the cars off: do they move in a straight line?
2 Are you truly symmetrical? Make accurate measurements of various parts of your body, such as your finger length, foot length, arm length and leg length. Are they the same on both sides? What about strength, speed and accuracy? Can you kick a ball better with one foot than the other? Can you throw, catch and write better with one hand than the other?