Episode 5: Wobbly Stick
Are you ready to compete in the world famous* Institute of Physics balance-off? Discover the lost art of stick balancing and explore inertia in episode 5 of Do Try This at Home.
Before you start:
These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.
What you’ll need:
- A stick/ broom/ something long-ish (over 50cm) and straight
- A lump of modelling clay/ plasticine/ blu-tack etc
What to do:
In this activity you’ll be trying to balance long sticks on your fingertips. To minimise any breakages in your home we recommend trying the trick outside if possible, or clear yourself as large a floorspace as you can away from valuables!
- To set up the demo, place a lump of modelling clay about the size of your fist around the stick, close to one end. Now you’re ready to challenge your family to a balance-off.
- First, hand them the stick with the modelling clay end at the bottom and ask them to try balancing the stick on the tips of their fingers. Maybe time yourselves, to see who can balance the stick the longest.
- Now turn the stick over so the modelling clay is on the top and try balancing again. Does it make a difference to how long you can balance it for? It should be much easier the further the modelling clay is from your hand.
What to talk about:
- Which way up was your stick when you balanced it for the longest?
- Try holding the rod at one end and waving it from side to side. Is it easier to move with the clay end next to your hand or far away?
What’s going on?
For the stick to stay balanced you need to be able to keep your hand directly underneath the heavy blob of clay. When things are perfectly balanced, gravity is pulling the heavy blob (or mass) straight downwards while your hand is pushing straight upwards to counteract it.
But this can’t last. As soon as the rod tips over slightly, these two forces are no longer lined up. Gravity is still pulling straight down on our mass, which makes the stick tip over more and more, faster and faster. To rebalance the stick you have to move your hand directly underneath the mass so the forces line up once more.
With the mass at the top, the stick tips over more slowly. It has to move further and that takes longer than with the mass at the bottom, giving you more time to move your hand. But why?
The word for how difficult it is to start (or stop) something is called inertia. The harder it is to start something moving the more inertia it has. You will probably have come across this when playing in the park, which until recently was full of moving objects called children. Think of pushing someone on a swing – the heavier they are the harder it is to get them started. Still in the playground but on to the roundabout: if someone wants you to push them it’s much harder if they stand near the edge. The inertia of an object moving in a circle depends both on its mass and how far it is from the centre
For our stick and blob of modelling clay, the further the lump of modelling clay is from your hand, the more inertia it has. The stick with most of the mass at the top tips more slowly, is easier to keep balanced and means that you know how to win a balance-off every time.
With the same materials you can investigate how the balance-ability of the stick changes if you:
- Try using larger or smaller lumps of modelling clay.
- Try moving the mass to different places along the stick, not just the ends.
If you want to go further, you can do some more experiments together testing out the inertia of different things. Maybe try using your wobbly stick to help you balance on one leg!
Did you know?
Tightrope walkers use the physics in this experiment when balancing. By holding a long pole as they walk along the rope they have more time to rebalance if they wobble.
*It’s not that famous… yet.
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