Balancing Stick

Which will fall?

 

 

 

Category - Force & Motion

Key Idea - Inertia

 

Purpose - Can be used to demonstrate inertia and centre of gravity. 

It's quite easy to balance the stick with the weight at the top!

 

But a much greater effort is required to balance the weighted stick this way around.
Nature -  Demonstration with student help
Materials -  A metre ruler or similar stick, large lump a plasticine 
Method - 
  1. Form the plasticine into a round lump close to one end of the ruler.
  2. Try balancing the ruler on one finger with the lump closest to your finger.
  3. Now try again with the lump away from your finger.
  4. You should find that the ruler is much easier to balance when the plasticine is at the top of the stick.
Safety - Nil
Explanation - Two different principles come into play to make this demo work.  The first is inertia.  The weighted end has greater inertia, so when it is at the top it momentarily resists the sideways falling, giving you time to compensate.

The other is the centre of gravity.  As the stick wobbles around, and you try to compensate, it is rotating around it's centre of gravity which is off centre in this case.  When the weight is at the top and the centre of gravity high on the stick, the end on you finger is able to move a large amount without effecting the top so much as to over balance.  When the weight is at the bottom only a small movement will cause the top to swing wildly, making it difficult to get under it again.
The same effect can be observed by comparing the ease of balancing a metre ruler, a 30cm ruler and a pencil.  The longer the object (and therefore the higher the centre of gravity), the easier it is to balance. 
Notes - The same effect can be observed by comparing the ease of balancing a metre ruler, a 30cm ruler and a pencil.  The longer the object (and therefore the higher the centre of gravity), the easier it is to balance.