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Wednesday, January 18, 2012

Looking for the Gravity Center

Yet on this video, the forks trick.

What we need:

  • 2 forks alike,
  • 1 toothpick or skewer.
How to:
  1. Place the forks  like the illusionist in the movie, otherwise you will never make it;
  2. Place the  toothpick or skewer between the fork tines, in the middle and up;
  3. Place our finger on the toothpick tip, like in the video
  4. Try to do the same you did on step 3 placing your finger on the opposite side.
NOTE: We didn't have access to a toothpick thick enough to fit perfectly the forks, so we used tape to hold them one to another. This doesn't affect the demonstration.

What happens?
In step3 the forks stay on the tip of your finger in a "falling" position.
In the step4 the forks moves in an attempt to find its balance.

Before anything else we need to understand 2 things:
  • Center of mass - this position is defined mathematically. It's the result of an external force acting on the object. This center of mass isn’t necessarily the geometric center of the object.
  • Center of gravity, it’s the point which we must apply the force to balance the force of gravity. Imagine a tray, if we want to balance it with just one finger, and assuming the center of mass is at the geometric center, center of gravity is exactly the point where we must apply the force to keep it still but suspended in the air without rotate or fall.

So, what determines the balance of a body is called the location of the center of gravity, in other words the "point of application" of the gravitational force (weight).

Returning to the forks, they are definitely in equilibrium, it's indifferent if we use a finger or any other object (for purposes of this magic trick we will say that our finger has paranormal abilities, but in fact physics takes care of it). Where is the center of gravity? The center of gravity is somewhere between the extremities of the forks.

Some daily examples:

  • The equilibrist uses the rod to keep the center of gravity at place, it's important that the center of gravity is above the wire; - The stick helps to restore balance.
  • When you grab a large/heavy object automatically you change your body position. This is due to the fact that our body has a fixed center of gravity, when you add more weight/volume to the equation, the body has to readjust the new center of gravity.- Body may assume incorrect positions when transporting heavy loads. 
  • Notice that, when a baby is learning to walk, often falls forward or sideways, to us this doesn't make much sense, but the baby is trying to learn where is his balance.- The balance is learned.
  • Finally, notice Cristiano Ronaldo, during the games, bends his knees and sudden change his direction, without falling, why? Among other things, because when he bends his knees he makes his center of gravity lower so he can get more balance. The same goes for race cars. A car with a lower chassis is a faster car, why? Because the center of gravity is lower and the car becomes more stable in tight turns and can therefore go faster. - lowering the center of gravity increases the balance.

Try to find other systems in weird balances.

Et voila!
Balance in your fingertips!


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