This simple experiment which is very easy to make teaches kids about the concept of center of gravity. All objects that have mass, also have a center of gravity. In sports, announcers talk about center of gravity when they want to discuss balance. This experiments demonstrates balance and center of gravity in a fun way.
Items Needed for Experiment
- Armature wire or coat hanger wire
- wood pencil
- metal washers
- wire cutter
Step 1: Find the materials needed for the experiment. We used a thin aluminum armature wire for the experiment wire. It is very light (which helps with distributing loads), it bends very easy but it holds its shape extremely well. Places like Hobby Lobby or Amazon carry this type of wire. It is typically used by sculptors who use it in large sculptures so it holds its shape. Wire from an old coat hanger also works, but it will be more difficult for the kids to shape the wire.
Materials needed for the center of gravity experiment.
Step 2. Have the kids take the pencil and try to balance it on their finger. If they place their finger in the middle of the pencil, they be able to balance the pencil laying horizontally. Now have them try to balance the pencil vertically on the tip of their finger. Is this possible?
Step 3. Cut a length of of wire that is slightly longer than the pencil. Adults, must help with this step or have the wire pre-cut for kids.
Step 4. Wrap the wire around the pencil several times, near the center of the wire to secure it in place.
Step 5. Attach several metal washers to the wire at each end. Wrap the wire through the center of the washers to secure them in place.
Step 6. Try to balance the pencil on top the edge of a table or on the tip of your finger. Keep in mind that you will need a low center of gravity to make the pencil balance.
By lowering the washer weights below the point of balance, it is easy to balance the pencil from its tip on a very small object, in this case the end of a pair of pliers. It is also easy to balance vertically on the tip of a finger.
Step 7. Try to balance the pencil on other objects by moving around the wire and weights. The pencil appears to defy gravity. Try balancing the pencil on the flat ground by spreading out the wires and lowering the weights to lower the center of gravity. Can you balance the pencil vertically on flat ground?
The pencil appears to defy gravity by moving around the wires and washer weights to change the objects center of gravity.
Every object with mass has a center of gravity. Think of the object's center of gravity this way. If you were to place a pin directly in the object's center of gravity and spin it, it would spin without wobbling. Calculating a center of gravity is used in practice by engineers when calculating forces on objects and when calculating the moment of inertia of a structural member.
When the kids try to balance the pencil horizontally on their finger, it may take a few tries, but it is not very difficult. When they try to balance the pencil vertically, it is easy to see it is impossible.
With the weights of the washers, it is easy to move around the object's center of gravity. This allows us to balance the pencil in many ways that seem to defy gravity itself. Think of hanging a V-shaped object from a wire. This is similar to the pencil with wires hanging down to its sides. It is easy to balance. When the kids tried to do the same thing on flat ground, they probably thought it was possible. Many of the kids may have been very close, but were not quite able to balance the pencil. On flat ground, we are not able to lower the center of gravity below the tip of the pencil, so it becomes nearly impossible to balance. We can get close, but the center of gravity is still above the resting point so there is a moment force making the object tip. Going back to the V-shape object and the wire analogy, take the same V-shaped object and bend it slightly upward and try to balance it on a wire. It is impossible!
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