Lesson 2 of 5 · Free fall & support
Why astronauts float
7 minute lesson · 3 checks along the way
Your goal for this lessonUse shared free fall to explain floating, and distinguish gravitational motion from the effects of air, floors, and engines.
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A feather tests an everyday assumption
On Earth, a feather usually drifts down more slowly than a hammer. That observation includes the effects of air. During Apollo 15, David Scott released a hammer and a feather together on the Moon. With essentially no air resistance, they reached the ground together within the demonstration’s accuracy. Different masses did not require different gravitational accelerations at the same location.
A more massive object experiences a larger gravitational force, but it also has more inertia: more resistance to acceleration. In the ideal model, those two effects match. Objects released with the same initial motion in the same gravitational field follow the same motion regardless of their . Air resistance, a parachute, or a running engine changes the conditions of the comparison.
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What your feet are telling you
Standing in a room, you are pulled downward by Earth and pushed upward by the floor. The floor prevents you from freely following gravity. The pressure you feel beneath your feet comes from that support. A bathroom scale responds to the force between you and the scale, which is why its reading is not a direct detector of whether gravity exists.
Imagine a sealed experimental capsule and a loose object falling together with air drag neglected. Both accelerate under gravity, so the object does not immediately settle onto the capsule’s floor. Relative to the capsule it can appear to float. This is a thought experiment about shared motion; an orbiting spacecraft allows that condition to continue without quickly reaching the ground.
The spacecraft is falling too
Inside a coasting spacecraft in Earth orbit, the crew, loose objects, and vehicle are all following nearby paths under gravity. They fall together around Earth rather than being supported by a stationary floor. Earth’s pull remains substantial at low orbital altitudes. The spacecraft’s forward motion and curved path, which we investigate next, keep it from simply dropping straight down.
Microgravity describes the almost weightless environment experienced inside such a vehicle. It does not mean every gravitational force is nearly absent. Small disturbances from air drag, vibrations, equipment, and differences in gravity across the spacecraft prevent perfect, identical free fall. Scientists measure and manage those effects when an experiment needs an especially quiet environment.
Apollo 9’s lunar module Spider above Earth on March 7, 1969, photographed from the command module. This was an Earth-orbit test, separate from the later Apollo 15 hammer-and-feather demonstration on the Moon.
Look closely
The spacecraft looks suspended above the clouds. Can a single photograph tell you that it is motionless or beyond gravity?
See what to notice
A still image freezes a moment of an orbital flight. During unpowered coasting, the spacecraft and its occupants fall together around Earth. Their shared motion explains apparent weightlessness inside; being above the clouds does not turn gravity off.
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Work through a decision
Change the conditions, change the prediction
Suppose a spacecraft begins an engine burn while an untethered instrument is floating inside. The engine now pushes on the vehicle; the instrument does not automatically receive that same push. Their relative motion changes until a wall, restraint, or another interaction transfers force to the instrument. Floating therefore depends on the situation, not a permanent property gained by crossing the atmosphere.
When you interpret a demonstration, list the forces and starting conditions before comparing results. Were both objects released together? Was there air resistance? Was the vehicle coasting or accelerating under thrust? A feather landing later in air does not overturn the vacuum result. It reveals an additional interaction that the simpler model deliberately left out.
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Put it together
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