Weightlessness

Weightlessness

A spacecraft on its path is in a constant state of free fall.

Physics

Keywords

weightlessness, gravitation, free fall, microgravity, cosmic speed, circular path, elliptical path, hyperbolic path, inertia, ellipse, hyperbola, space shuttle, spacecraft, linear motion, orbit, velocity, space research, inertial frame, mechanics, outer space, physics

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Scenes

Space shuttle

Cause of weightlessness

  • free fall
  • tangential linear motion

Weightlessness in the spacecraft

Paths

  • descending path - The tangential speed is less than the first cosmic speed (about 8 km/s).
  • circular path - The tangential speed is equal to the first cosmic speed (about 8 km/s). The spacecraft is moving forwards just fast enough to stay on the circular path. It is actually ´falling´ around the Earth. The long-lasting free fall explains weightlessness.
  • elliptical path - When the tangential speed is greater than the first cosmic speed but less than the second cosmic speed (about 11 km/s), the path of the spacecraft becomes elliptical.
  • hyperbolic path - When the tangential speed is greater than the second cosmic speed (about 11 km/s), the spacecraft breaks free from the Earth´s gravitational pull on a hyperbolic path. Thus the second cosmic speed is also called ´escape velocity´. (If the spacecraft reached the third cosmic speed - about 42 km/s - , it would leave the Solar System).

Animation

  • free fall
  • tangential linear motion

Narration

A spacecraft moves along a circular path around the Earth. Its motion consists of two components: tangential motion and free fall. These components result in circular motion.
Since the spacecraft is in a constant state of free fall, it is actually ´falling´ around the Earth, which explains weightlessness.
The speed which keeps the spacecraft on a circular path is called first cosmic speed, which is about 8 km/s.

When the tangential speed is greater than that, the path of the spacecraft becomes elliptical. According to Kepler´s 2nd law of planetary motion, at the apogee, when the spacecraft is farther from the Earth, it slows down, then at the perigee, when it is near the Earth, it accelerates.

When the tangential speed is lower than the first cosmic speed, the spacecraft starts to descend.

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