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TwiddleLab / Physics / Gravity and Orbits

Weightless Means Falling

Lesson 5 of 10 · 7 min · Free, no account needed

What you will learn

  • At the space station gravity is still about 88% as strong as on the ground.
  • Weightless means in free fall: you and everything around you fall together, so nothing pushes on you.
  • Pull at height h: (R / (R + h))² of the ground value, the inverse square measured from Earth's centre.

Start the lesson

How the lesson goes

  1. Gravity at the ISS is about 90% as strong as on the ground. So why do astronauts float?

  2. Build a tower to the ISS and stand on a scale at the top. What does it read?

  3. So what is different on the station? Cut the cable of this lift and watch the scale.

  4. Weightlessness is free fall

    A scale shows how hard you push on it. When you and the scale fall freely together, nothing pushes, and it reads 0: you are weightless.

    Gravity is not gone. The station and the astronauts fall round Earth together, all the time.

    Pull at height h: (R / (R + h))² of the ground value. Simplified model: no air drag inside the station.

  5. Zero-g planes give you weightlessness on Earth. Draw the plane's height while its passengers float.

    It climbs at 100 m/s when the float starts at 5 s. From then on it moves only under gravity.

    In free fall the height is y = 7500 + 100t - 4.9t² (t counted from the start of the float). That is a parabola with its top when the upward speed is used up, after about 10 s.

  6. Game: Zero-g flight. Fly the plane so the passengers float as long as possible.

    The stick sets the plane's acceleration up or down. The air is bumpy.

  7. Math: which model of the pull fits the measured dots?

    The pull goes as 1 / r² where r is the distance from Earth's centre. At height h, r = R + h, so the share of the ground value is R² / (R + h)².

  8. Tap every scene where the person is weightless.

    Weightless means only gravity acts on you. In a car, an accelerating lift or at a skydiver's top speed, a seat, a floor or the air pushes back.

  9. Back to the hook. How strong is gravity where the station flies?

    At 420 km up, r = 6,791 km. The pull is (6371 / 6791)² of the ground value, about 0.88.

Previous lesson: Orbit Speed and Period · Next lesson: Ellipses and Kepler's Laws · Part of the course: Gravity and Orbits

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