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Gravity is the force that pulls objects toward the center of the Earth. An object’s center of gravity is the average location of the weight of the object.
If you have a wooden ruler you can balance it on your finger at the 6 inch mark. The ruler has a uniform shape and it’s weight is evenly distributed along its shape. It is easy to determine that the 6 inch mark, right in the middle, is the center of gravity for the ruler. There is an equal amount of weight on each side of the 6 inch mark; the gravitational pull on both sides of the rule is equal – so the ruler will balance.
If you have an object like a person trying to balance on a balance beam, then it is a little trickier to figure out where the center of gravity is located. It will be a little different for each gymnast depending on how their weight is distributed on their body. Each gymnast learns the location of their own center of gravity and knows to keep that point centered over the balance beam. If that point is not centered, the gymnast will fall off.
In today’s experiments you will find the center of gravity for objects that are not as easy as a ruler, you will find the center of gravity for a pop can and a fork.
What You Need:
- an Empty Soda Can
Here is a video that will show you exactly what to do to balance a pop can:
I tried the pop can and it really did work. It only took a couple tries too. It is a little harder to tap it and make it go in a circle.
Balancing the two forks is a little harder.
What You Need:
- Two Identical Forks
- Salt Shaker
- Empty Pop Can
First, stick a toothpick in the center hole of a salt shaker. This will act as your balance point. Now take the two forks and press the tines of the two forks together, making them into an “L” shape. Hold a toothpick in your hand and try to balance the forks on the toothpick. Once you get the forks balanced, transfer them from the toothpick you are holding to the toothpick stuck in the salt shaker. The girl in the video makes it look easy. Not easy!
Here are some websites and books that will help you understand and play with gravity:
- Physics4Kids: Gravity
- StarChild: What is Gravity
- Fetch: Set it Straight
- ZOOM: Weightlessness
- Tryscience: Gravity in Action
- PBSKids Fetch: Skydiver
- PBSKids Fetch: Stack ‘Em Up
- PBSKids Fetch: Throw Your Weight Around
- The Guardian: Perfect Balance Nails
- Bill Nye Falling for Science
- Bill Nye Weightless Clothespin
- You can find the directions for this experiment at Steve Spangler Science: Balancing Forks and in Steve’s book Balancing Forks (page 51).
- Gizmos and Gadgets Chapter “All Fall Down” about Gravity (Pages 49-67).
Words to Know:
Gravity – The force that pulls everything toward the center of Earth; the force of gravitation on Earth.
Center of Gravity - The average location of the weight of an object.
Weight – A measure of the force of gravity upon an object
Balance – An even distribution of weight so that an object will remain upright and steady even if it is resting on a very small point.