Level: Elementary to Midddle School
Time involvement: 10 -15 min.
Mention the term zero gravity and images of science fiction movies or possibly the International Space Station come to mind. People gently tumbling about with no sense of up or down and a spilled glass of water looking like wiggling clear jelly, is not far from the truth. In reality, people in a falling elevator would appear much the same with the exception of the terror on their faces. Contrary to popular belief zero gravity does not exist anywhere. Because the gravitational field of every body is technically without limit there is no such thing as “perfect zero gravity”.
However, in many places in the universe, given the vast distance between stars, the gravitational pull of any body is very slight. The ISS is in an earth orbit varying from about 200 miles to 250 miles. If engineers could build a tower 200 miles tall the occupants at the top would barely notice the decrease in the earth’s gravitational pull. The occupants of the ISS are moving at about 17,000 miles per hour. (About 4.5 miles per second) One trip around the earth takes about 90 minutes. At this speed the centrifugal force of the curved path exactly matches the gravitational force. Hence, the feeling of “zero gravity”. In practice NASA prefers the term “microgravity” as more technically accurate.
Now, for the “detector” part. Looking like a failed grade school art project, the detector actually works. Notice that the construction is extremely simple. Take a plastic cup, put a small hole in the bottom. Obtain two weights of about an ounce. The large metal nuts shown are an easy choice, and they have a hole allowing the attachment of a rubber band. Select two rubber bands of about 2-3 inches in diameter. Select lighter strength ones. Cut them open. Tie a weight on the end of each. The next step requires a bit of careful attachment. Suspend the weights just over the edge of the cup. Feed the loose ends through the hole in the bottom of the cup. Using a paperclip, toothpick or similar item as an anchor draw the rubber bands lightly taut. If there is too much tension the nuts will be pulled into the cup during the normal 1 g. If there is not enough tension the nuts will not move into the cup during the moment of “zero g”. Secure the ends of the rubber bands to the toothpick, paperclip, etc. or just tape them in place.
Time for the test run. Hold the cup at eye level or higher. The action occurs in a fraction of a second so you must closely focus your eyes on the weights. Trying not to tip the cup, allow it to fall a foot or two. With practice you should be able to catch the cup in a vertical position. If the tension on the weights is correct the moment of falling will create “zero gravity” for the cup and weights, not you! The weights will now be lying in the bottom of the cup. Take a bow, you have created “zero gravity” and can prove it!
The “g” or gravity meter (sometimes called an accelerometer) is not a common aircraft instrument but not rare either. Most military fighter jets have one, airshow airplanes all have one, and many experimental aircraft will have one. The g meter in the photograph reads one g. Much like a maximum-minimum recoding thermometer this g meter saves the maximum and minimum force measured. The knob at the lower left is used to reset the hands back to 1 g. The purpose is to measure and record the stress placed on the airframe to insure that flight maneuvers will not exceed the design strength. A small private aircraft might be built to withstand 3-4 positive g’s and 1-2 negative g’s. Military fighters might be built to withstand up to 10 g’s without structural failure. Unfortunately, the human body cannot withstand more than 3-4 g’s for a sustained period without “blacking out” due to the blood being drained from the brain. The solution to high g survival is a g suit that basically squeezes the abdomen and legs to minimize blood “pooling” in the lower parts of the body.
A training maneuver called “steep turns” with a 60 degree bank angle will subject the aircraft and all passengers to a sustained two g force. Not dangerous but an unusual feeling for the first few times. By flying an aircraft through a special parabolic arc “zero gravity” can be sustained for 30 or 40 seconds. Cockpit items must be secured first to prevent them from “floating around”.
Discussion: The above explanation covered many points. The possibility of actually experiencing different g forces is usually reserved to carnival rides unless a person has an airplane ride with a willing pilot. An interesting exception is the experience of skydivers. Because the “jump” is made in the atmosphere the drop is not a “pure free fall”. Atmospheric resistance, (drag) will cause the acceleration to gradually diminish. After a few seconds “terminal velocity” will be reached. If a recording g meter were being carried it would show that the “zero” g forces soon returned to a normal 1.0. However at some point the parachute will open (hopefully) and briefly the g forces will exceed 1 g and then return to normal.
An amazing record setting jump on October 24, 2014 was made by Alan Eustace (beating out Felix Baumgartner), wearing a space suit, from an altitude of 135,000 feet over New Mexico. In the extremely thin atmosphere he reached a speed of 821 mph, breaking the sound barrier. His descent to earth lasted 15 minutes.