| DROP TOWERS |
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On Earth, microgravity experiments are often conducted in drop
towers. As the name suggests, experiments are literally dropped
down a shaft to achieve a few seconds of microgravity. The experiment
hardware becomes nearly weightless because it is in "free-fall."
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The experiments fall from rest with the acceleration of gravity,
following the equation below:
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where d is the distance the object has fallen, g
is the acceleration due to gravity, and t is the time that
the object has fallen. On Earth, g is about  .
The approximate distance an object falls on Earth is shown below.
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| Time (seconds) |
Fall (meters) |
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| 0 | 0 |
| 1 | 5 |
| 2 | 20 |
| 3 | 45 |
| 4 | 80 |
| 5 | 125 |
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For true "free-fall," where the equation applies, gravity
must be the only (net) force acting on the object. Air drag commonly
prevents this, but is avoided in drop towers by aerodynamic design
or evacuation of the air.
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| ON ORBIT |
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Microgravity conditions are achieved on spacecraft by allowing
them to "free-fall" toward the Earth on a circular
path called an orbit. This allows astronauts on board spacecraft
to perform microgravity experiments lasting hours, days, or weeks.
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To remain in orbit, a spacecraft must travel at a very high velocity.
The required velocity is dependent on gravity, and decreases with
increasing altitude (i.e., distance) as shown:
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where v is the orbital velocity, R is the radius
of the orbit, and g is the local acceleration of gravity.
acceleration. The universal gravitational constant and mass of
the Earth are respectively:
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For reference, the Earth's radius is 
meters. |
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For a typical Space Shuttle altitude of 300 kilometers, the orbital
velocity is nearly 28,000 kilometers/hour.
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LEARN MORE ABOUT MICROGRAVITY ON THE WORLD WIDE WEB AT:
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NASA Headquarters
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http://microgravity.msad.hq.nasa.gov/
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| NASA Glenn Research Center |
| http://www.grc.nasa.gov/
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