Science Questions

Why do things float on the space station?

Mon, 15th Jun 2015

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TERRY72763 asked:

If gravity holds the space station in orbit then why is it 0g inside?


If gravity can hold the space station in orbit, than why does the supposedly from space tele- visit always shown as 0g. Wouldn't gravity be present in an orbiting body? Even minutely! the why don't the interviewed bodies fall towards the gravity holding the spacecraft in orbit?


Science humourist Dave Zobel had a go at answering Terry's question

Dave - Why do the astronauts float about? If they just put on heavy boots, wouldnít they be able to stomp around like the rest of us? The brief answer Terry, is the same reason that when you jump off of a very high building with your keys in your hand Ė which I donít recommend Ė as you are descending to the Earth, your keys sort of float up from your hand. They donít stay anchored in your hand and the things that are in your pockets donít stay anchored there. and the reason for that is because you and your keys, and everything else are all falling at the same time and at the same rate, well thatís what's going on with the International Space Station and really, anything else in orbit. You are falling not to the Earth but around the Earth, but itís the same idea. And everything is falling. So, itís all falling at the same rate. Thatís also the reason that when astronauts train in the airplane known colloquially as the vomit comet where they go up and then come crashing down towards the Earth and then at the last moment, pull out and go up again and do that over and over, and over until everybody has had the chance to see what everybody else had for breakfast. There's that same weightless period and that gives them 20 or 30 seconds to be able to float around and feel what itís like to be in the International Space Station.



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The space station in orbit is in "Free Fall" so everything in it experiences the same gravitational force and acceleration. The space station and everything in it is falling together, so there is no relative acceleration--no relative force. This is the same reason you feel weightless at the very top of the arc on a swing, or when the roller-coaster plunges, or if you ever happen to go sky diving.

The weight or "force of gravity" that you feel in your feet when standing isn't really the force of the Earth pulling you down--It is the equally strong force of the ground pushing you back up. chiralSPO, Wed, 10th Jun 2015

Very good! When you posted that I was in the middle of writing a full treatment of this explanation. Since it took some effort I'd like to post it rather than toss it out. In any case it merely adds to what you said rather than contradicts it in anyway.

My response to the question:

All of your questions can be resolved if you understand a few things about gravity (In what follows we'll assume that the mass of the source of gravity is so large compared to the object orbiting it that the planet can be considered to be at rest).

1) Gravitational force is proportional to mass - One of the important thing to know about the gravitational force acting on a body is that the force is proportional to the mass of the body on which its acting. If the source of gravity (I'll call it a planet) is either a point object, located at the origin of the coordinate system we'll use to describe the motion, or the mass density is spherically symmetric then the force due to gravity on a particle of mass m which is outside the body has the value Fg = GMm/r2n where n is a unit vector pointing from the particle to the source of gravitation (again, I'm assuming the source is spherically symmetric).

2) Acceleration of a body in a gravitational field is independent of the mass of the body - Since force on a body of mass m and acceleration a is F = ma we can see that force is proportional to mass. It follows this that the acceleration of a body in the gravitational field of a planet is independent of its mass so long as the dimensions of the body are small enough such that any differences in the gravitational force (i.e. gradients in the field) over the body can be neglected, and we make our observations over a small enough interval of time. When the acceleration is due to gravity the force is referred to as gravitational acceleration and is represented by the vector g. This follows since when we equate the two expressions we get

F = ma = GMm/r2n


g = GM/r2n

3) If an object is moving at a speed, the instantaneous direction being parallel to the surface of the gravitating body, then the gravitational force will equal the centrifugal force that is required to force a body to move in a circle then the body will be in a circular orbit around the source.

4) Since the acceleration of any body moving in a gravitational field is independent of its mass it follows that if two bodies have near each other and start off with the same speed and direction then they will remain near each other. They wonít move relative to each other. This will hold of any number of particles, people, hotdogs, bottles of Tang, etc. Since this fits the definition of an inertial frame of reference it then follows that the interior of the space shuttle is a weightless environment. There is effectively no gravitational field inside the shuttleís cabin. To be precise itís referred to as a microgravity environment since the gravitational field of planets have gradients in them (also known as spacetime curvature). You can learn more about microgravity at NASAís website at:

PmbPhy, Wed, 10th Jun 2015

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