Astronauts float in orbit because they are in continuous free fall around Earth. Gravity is still strong at the altitude of the International Space Station, so orbit is not a place where gravity disappears. The spacecraft, astronaut, and everything inside are all falling toward Earth together while also moving fast sideways.
This shared motion makes the astronaut feel weightless.
Key Facts
- Weight is the gravitational force on an object: W = mg.
- Apparent weight is the support force you feel, such as a normal force from a floor or seat.
- In orbit, gravity provides centripetal force: Fg = mv^2/r.
- Orbital speed for a circular orbit is v = sqrt(GM/r).
- Free fall means gravity is the only significant force acting on the object.
- Near the International Space Station, gravity is about 90% as strong as at Earth's surface.
Vocabulary
- Weightlessness
- Weightlessness is the condition of having little or no apparent weight because no support force is acting on you.
- Free fall
- Free fall is motion in which gravity is the only significant force causing acceleration.
- Apparent weight
- Apparent weight is the force a scale or surface exerts on you, which is what you physically feel as weight.
- Orbit
- An orbit is a curved path around a body caused by forward motion combined with gravitational attraction.
- Centripetal acceleration
- Centripetal acceleration is acceleration directed toward the center of a circular path, given by a = v^2/r.
Common Mistakes to Avoid
- Saying astronauts float because there is no gravity is wrong because Earth's gravity is still strong in low Earth orbit and provides the force that keeps the spacecraft moving in a curved path.
- Confusing weight with apparent weight is wrong because weight is the gravitational force W = mg, while apparent weight is the support force felt from a surface or scale.
- Thinking an orbiting spacecraft is not falling is wrong because it is constantly falling toward Earth, but its sideways speed makes it keep missing the ground.
- Assuming weightlessness requires being far from every planet is wrong because weightlessness can happen whenever objects share the same free-fall acceleration, even close to Earth.
Practice Questions
- 1 An astronaut has a mass of 70 kg. If g = 9.8 m/s^2 at Earth's surface, what is the astronaut's weight on the ground?
- 2 At an orbital altitude where g = 8.7 m/s^2, a 9000 kg spacecraft is in free fall. What gravitational force acts on the spacecraft?
- 3 Explain why a scale inside an orbiting spacecraft would read nearly zero for an astronaut standing on it, even though Earth's gravity is still acting on the astronaut.