In a video from an orbiting station, someone releases an object and it stays nearby. Feet do not press on the floor; objects do not drop onto a table. It looks as though Earth has stopped attracting them. Yet Earth’s gravity is what keeps the station in orbit. How do these facts fit together?
Astronauts float because the person, station and loose objects are falling together under gravity, so the floor need not support them from below. Gravity has not disappeared in orbit. The familiar load on a support is missing, while gravitational force remains. NASA: why objects float in orbit.
The familiar feeling starts with the floor
When you stand in a room, Earth pulls your body down, but the floor prevents you from falling freely. Your feet push on the floor, and the floor pushes up on your feet. That contact force is the normal force. Even a rigid support deforms slightly and pushes back against its load. OpenStax: how a support force arises.
A force scale responds to this interaction. It has no separate sensor for “how much Earth gravity is here”: contact with the body loads its spring or force sensor. A household device converts force into a kilogram reading using its calibration. Our diagram instead displays force directly in newtons, with the person’s mass given separately.
Terminology needs care. In many English textbooks, weight means the gravitational force on a body; the support-related quantity is called apparent weight. The original Ukrainian article follows the school convention in which “weight” is the body’s gravitationally caused force on a support or suspension. The support’s reaction acts in the opposite direction on the body. These are different forces on different objects, although their magnitudes are equal as a contact pair. Earth’s gravitational force is a separate interaction. Check definitions when comparing languages or sources. Ukrainian Physics textbook, §12, p.75.
When the floor falls with you
Imagine a small closed cabin, a person and a sphere initially at rest relative to one another. Now they start to fall freely: this is a thought experiment without air resistance or other external influences. In a small region, Earth’s gravity gives them almost the same acceleration.
The cabin no longer prevents the person from falling: it falls alongside. No force is needed to press the person against the floor. An ideal unloaded scale reads zero. If the sphere was simply released without a push, it does not rapidly drop relative to the person: both move almost alike.
This is apparent weightlessness: in our idealization, the body does not load a support or suspension. Its mass remains, Earth still attracts it, and person and cabin accelerate together. Falling together explains the unusual view inside. NASA Glenn: free fall and a scale reading.
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How a scale can read zero when g is not zero
Let m be the person’s mass in kilograms, g the local gravitational acceleration and a the magnitude of the cabin’s downward acceleration, both in m/s². Acceleration describes a change in velocity over time. Write the normal force on the person as N and choose upwards as positive. With only support and gravity acting:
N − mg = − ma,; N = m(g − a).
For a = 0, N = mg: the support balances gravity. In joint free fall, a = g, so N = 0 even though mg is not zero. For illustrative values of m = 60 kg and g = 9.8 m/s², the gravitational force remains 588 N.
This is a local vertical model in an almost uniform field. For maintained contact we use 0 ≤ a ≤ g; if the floor accelerates down faster, it moves away from the person rather than producing a “negative normal force”. The equation does not calculate an entire orbit.
In orbit, falling does not end at the surface
An ordinary short vertical trip eventually reaches the ground. In orbit, the station also has enough sideways velocity: as gravity curves its motion towards Earth, the rounded surface remains below its path. The station falls around Earth, rather than hovering above it. On a circular orbit its altitude can remain constant while its velocity keeps changing direction. That change of direction is also acceleration.
The person inside already has almost the same orbital velocity. Gravity changes that motion much as it changes the station’s motion. The person therefore does not require a floor continuously pushing upwards. This explanation needs no engine permanently holding the spacecraft up from below. NASA: how free fall becomes orbital motion.
The distance from Earth’s centre has not increased enormously. At an illustrative altitude of 400 km, a simple spherical model gives gravitational acceleration of about 8.7 m/s², roughly 89% of its surface value. That is plenty to change the station’s motion strongly. Here 400 km is a chosen teaching altitude, rather than a current report of the ISS’s orbit.
Compare the three states. First predict what changes when support gives way to joint free fall near the surface: g itself, or contact with the floor?
Gravity acts. Is the support loaded?
Predict whether g changes when the cabin starts falling freely. Choose a state and compare gravity with the scale reading. The keyboard arrows change the selection.
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Where 89 percent comes from
Outside a spherical, nonrotating Earth, gravity weakens with the inverse square of distance from the centre, rather than height above the ground. Let R ≈ 6371 km be Earth’s radius, h = 400 km the altitude and g_(0) ≈ 9.8 m/s² a rounded surface reference. Then:
(g(h))/(g_(0)) = (((R)/(R + h)))^(2); = (((6371)/(6771)))^(2); ≈ 0.885.
This gives g(h) ≈ 8.7 m/s². For the same mass of 60 kg, gravitational force would be about 521 N, rather than zero. The ratio of radii is dimensionless; express both radii in the same units. Earth’s rotation, flattening and local field variations are omitted. NASA: Earth’s radius and parameters, OpenStax: the law of gravitation.
A real station only approaches the ideal
A real spacecraft experiences small additional influences. The thin upper atmosphere slows the station; fans and pumps run, people move and engines sometimes fire. Gravity also differs slightly across a large spacecraft. Objects can therefore drift slowly and experience vibrations.
In this context, microgravity describes an environment with small residual relative accelerations and loads. It does not mean that Earth’s large gravitational g has become one millionth of its surface value. The quality of the environment depends on location, time and equipment activity. NASA: researcher’s guide, pp.42–43.
There is a simple limit to the explanation: a vacuum alone does not create weightlessness. If a body rests on a stationary support in a vacuum chamber, gravity and support loading remain. And if an astronaut pushes off a handrail, they gain motion relative to the cabin: weightlessness does not erase pushes or make mass zero.
Check the diagram: why can a scale read zero both near the surface and in orbit, despite different values of g? Because in both free states the person is not held up by a loaded support. A zero reading concerns contact force, not the disappearance of Earth or its gravity.
The astronaut therefore floats with the station in joint free fall. Return to the three states and follow just two forces: gravity on the person and the floor’s force on them. The next course question, about mass and its difference from weight, will reinforce this distinction.