Everyone needs some support to be balanced!
You don't have to be a tightrope walker to do balancing tricks, but you do have to understand the role of an object's centre of gravity. In the following activities, students explore the conditions necessary for balance in order to keep objects from toppling over and how we use these concepts in our daily lives.
- Identify ways in which balance affects our daily lives.
- List the conditions required for an object to be stable.
- Find the centre of gravity of an object.
The centre of gravity of an object could also be called its balance point. If you support the centre of gravity, the object will balance.
We sometimes use "centre of mass" and "centre of gravity" to mean the same thing. Technically, they are different. Centre of mass means "the average location of all the matter in an object". The centre of gravity is the average location of the force of gravity on an object. They're only different if the force of gravity is different on different parts of the object. How can that happen? The force of gravity gets weaker as you move away from the Earth, so a very, very tall tower might have a slightly smaller force of gravity at the top than at the bottom.
For these activities, we'll assume that we're doing experiments with small objects close to the surface of the Earth, so the centre of mass and centre of gravity are in the same place.
If an object has its weight distributed equally throughout, its centre of gravity is located at its geometric centre. If the object has an irregular shape, its centre of gravity may not be at its geometric centre. It is possible for the centre of gravity to be entirely outside the physical boundaries of the object. The centre of gravity of a football, for example, is in the empty space inside the ball, and the centre of gravity of a boomerang is off to the side of its inside curve.
If an object is not supported directly below its centre of gravity then the object will topple over. This explains why a wide stance helps you keep your balance. As you move your centre of gravity (near your belly button) to and fro, it stays above your support (your feet). A tall object with a high centre of gravity and a skinny support will fall easily because even a little sideways motion pushes the centre of gravity off the support.
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