Try this: Balance sticks

This is a nice little balance demonstration I picked up when I worked at Questacon, the National Science and Technology Centre.

You will need

• A ruler
• Some plasticine, bouncing putty, Blu-Tak or equivalent.
• Some other long skinny things, such as an umbrella or a golf club. Don't use things that might give you splinters.

Here's what to do:

1. Hold your hands out in front of you, with your fingers pointing forwards and your thumbs pointing up.
2. Lay the ruler on top of your hands.
3. Bring your hands together, like you were doing a slow motion clap.
4. While your hands are moving, do not try to grab the ruler with your thumbs.
5. Watch the ruler.

The first time I tried this, I was surprised to find the ruler didn't fall. Instead, my hands came together in the middle of the ruler, leaving it balanced on top.

You can try a couple of variations of this activity:

• Stick a lump of plasticine to one end of the ruler. Check if it still works.
• Try other objects, like a golf club or umbrella.

Whats happening?

Your ruler has a balance point, a spot that you can balance it on. The technical name for this balance point is the "Centre of Mass". Not surprisingly, for your ruler the centre of mass is right at the middle of the ruler.

When you were bringing your hands together, the object would stay balanced as long as the balance point was somewhere between your hands. You may not notice it, but when the objects are lying on your hands, your hands don't feel the weight equally. Although the total weight is always the same, the hand closest to the centre of mass feels more weight than the other hand. So the hand furthest from the centre of mass moves more easily. You will find that as you slide your hands, only one hand will slide against the ruler at a time and its always the one furthest from the centre of mass. This means that when your hands meet, it will always be at the centre of mass, so it doesn't fall.