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Taller: How does Gravity Affect a Person's Height?

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Source:
Author: Janice VanCleave

Problem

How does gravity affect a person's height?

Materials

• scissors
• 2-liter soda bottle with cap
• 5 empty plastic thread spools
• ruler
• string
• bowl large enough to hold the soda bottle
• tap water
• helper

Procedure

1. Have an adult cut the bottom from the bottle.
2. Remove any paper covering from the ends of the thread spools.
3. Cut an 18-inch (45-cm) piece of string.
4. Place the string in the bottle so that about 2 inches (5 cm) of string hangs out of the mouth of the bottle.
5. Secure the cap on the bottle, leaving part of the string hanging out.
6. Turn the bottle upside down, then thread the free end of the string through the holes in the spools so that they slide down the string and stand end to end inside the soda bottle.
7. Set the inverted bottle in the bowl.
8. Support the bottle in an upright position with one hand, and hold the string with your other hand so that the spools stand straight.
9. Compare the top of the top spool with the top of the bottle. Notice the position of each spool.
10. Ask a helper to fill the plastic bottle with water while you continue to hold the string up.
11. Again, compare the top of the top spool with the top of the bottle. Notice the position of each spool.

Results

Without the water, the spools stand tightly on top of each other. When the bottle is filled with water, the spools float upward, and there is some separation between spools. The top of the top spool is lower in the empty bottle than it is in the bottle full of water.

Why?

Without the water, gravity pulls the spools downward, causing them to stand tightly against each other. The water pushes up on the spools. This upward force exerted by a fluid on an object in the fluid is called buoyancy, and thus the water simulates a low-gravity environment that reduces the downward pull of gravity. With less downward pull, the spools are allowed to move around more freely. They do not move away from each other because of the connecting string.

The human backbone resembles the stack of spools in that the separate disks that make up the backbone are free to move apart, as are the spools. like the spools, the backbone has a cord, called the spinal cord, that runs through the center of the disks. Gravity pulls the disks down against each other. In space, the discs separate, and the backbone gets longer because gravity is not pulling it down. Astronauts in space are taller than they are on earth.

Let's Explore

What would happen if the spools were connected? Cut a strip of cloth about 2 inches (5 cm) wide and 1 inch (2.5 cm) longer than the height of the stacked spools. Attach the ends of the cloth to the top and bottom spools with duct tape. Repeat the experiment. The cloth strip represents ligaments (tough bands of tissue connecting the ends of bones) that limit the separation of the disks in the backbone. Science Fair Hint: Display the spools with the attached cloth strip along with diagrams and a printed copy of the results.

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