# Help my coworker understand gravity in space

It all started with astronauts on Fox News, being interviewed from the Space Station. A coworkers says to me “Wouldn’t it be cool to feel weightless like that?” I told him that, in fact, he has felt that way anytime he’s been on a steep rollercoaster, a quickly descending plane, a falling elevator, trampoline, or swimming pool. He refused to believe this.

His reasoning is that you get a knot in your stomach because you’re falling. And the astronauts just float around. I tried explaining that the astronauts are falling too, but he swears it’s not the same thing. He keeps saying that there’s no gravity in space. I ask what keeps the moon in orbit, and he says it’s because the moon is big (which I know to be irrelevant). He’s said that on the space station, he (a 270lb football player) would weigh nothing while I keep trying to explain that he’ll weigh roughly 250 lbs. And lastly, he says that there is gravity outside the station, but not inside it.

It doesn’t help that other, worsely informed coworkers keep passing by to drop stupid-bombs by saying things like “If a plane flies really high, the passengers will be weightless” , “There are Lagrange points where there isn’t any gravity”, “The space station is outside the gravitational field of the earth” and “You can’t calculate gravity, you can only see it’s effects…like the wind.”

These people have no understanding of force, velocity, acceleration, or a bunch of other Physics 101 concepts. So help me explain the following:

1. You’re not weightless on the space station.
2. It’s exactly like being in a plummeting elevator.
3. There’s gravity in space, inside the atmosphere, outside the atmosphere, in the space station, and outside the space station.

Some people can’t be helped. Don’t bother, it’s a waste of your time and annoys the cow-workers.

Send him here: http://spaceplace.nasa.gov/en/kids/orbits1.shtml

1. You are weightless in space (at least while in free-fall). Try standing on a scale and see. You still have mass but no weight.

2. A plummeting elevator is in free fall. You are in free fall with the elevator. We know from Newton that regardless of mass all things fall at the same rate (in a vacuum…air resistance can complicate things). As such, you are weightless in the elevator as long as it is falling. You can test this by standing on a scale in an elevator and watch you “lighten” a bit as it starts to go down. Of course it never goes so fast that you’d be weightless but if it went into free fall you would be.

Show him this page from Zero-G Corporation which takes people on plane rides which exploits these effects to simulate zero-g (our astronauts used to train in a similar fashion).

If you want to explain astronauts being in free fall try this:

Imagine you are on earth but it is perfectly smooth (no mountains) and airless (vacuum). Now throw a ball. It travels out a bit and lands on the ground. Normal stuff. If you trace the trajectory of the ball it describes an arc. Throw it harder and the ball goes further lengthening the arc out. In all cases it falls to the ground.

Now throw the ball really hard. If you throw it hard enough the arc will match the curvature of the earth. The ball will be falling to the earth but the earth will be continually curving away from the ball. As a result the arc never intersects the ground and you have just put the ball in orbit around the earth. Note the ball is falling the whole time. It just never meets the ground. This is the same thing our astronauts and satellites do.

1. Gravity in space is answered in #2 above. The ball is falling the whole time. It is curving around the earth. If there were no gravity in “space” all our satellites would just fly off into deep space. They are held in orbit by gravity (for that matter the earth orbits the sun because there is gravity in space and sun around the galaxy and so on…good thing gravity is there or we wouldn’t be here).

What? Just because the scale doesn’t register my weight doesn’t mean that I’m weightless. I’m a mass in a gravitational field, thus I have weight. You’re thinking of apparent weight.

[quote=Wikipedia “Weight”]
weight is a force that results from the action of gravity on matter: it measures how strongly gravity pulls on that matter."

[quote]

And anyway, I don’t have a scale. I turned it upside-down yesterday accidentally and it exploded from the weight of the Earth.

You had to go awfully far down the wiki entry to get to that sentence. Let’s go with something near the top:

"In the physical sciences, the weight of an object is the magnitude, W, of the force that must be applied to an object in order to support it (i.e. hold it at rest) in a gravitational field. "

Please note the words “at rest”.

[quote=“Chessic_Sense, post:5, topic:516581”]

What? Just because the scale doesn’t register my weight doesn’t mean that I’m weightless. I’m a mass in a gravitational field, thus I have weight. You’re thinking of apparent weight.

[quote=Wikipedia “Weight”]
weight is a force that results from the action of gravity on matter: it measures how strongly gravity pulls on that matter."

You are using the word weight in a way that is not commonly used. Weight is the force needed to keep you in in place relative to the environment around you. In the space station a chair does not need to exert a force on me to keep me in place.

Err…that is exactly what it means. A scale measures weight. If you stand on it and it reads zero it is measuring zero weight thus you weigh nothing there.

You have mass of course and that remains unchanged.

Think of it like this. Gravity on the moon is 16.5% of that here on Earth. On the moon if you stand on a scale you’d register as weighing 16.5 kilograms whereas on Earth you’d register as weighing 100 kilograms.

No, weight is the force needed to keep you in place relative to the gravitational field. More specifically, weight is the force exterted on an object by gravity. W=mg and all that. On the space station, are you saying that m=0 or that g=0? Because neither is true.

Only if you use it correctly. Standing on it while you both move is not using it correctly.

Define “moving”.

Sit really still in your chair for a second. Were you moving? Nope? Well, actually you were moving quite fast. Several hundreds of miles per hour as you rotate around the planet’s center, several thousand miles per hour around the sun, more around the galaxy and more with the galaxy through the universe.

So, on the space station standing on your scale are you and the scale moving? Yes and no. Relative to each other you are not moving. You are both in the same reference frame so the scale can measure your weight in that frame…which is zero on the space station.

From the reference frame of the Space Station, the astronauts are weightless. From the reference frame of the surface of the Earth, they have a weight nearly as large as their normal weight. But frankly, I’m not sure how useful it is to go all relativistic on this co-worker.

That is not what most people mean when they speak of weight.

But it is what physicists usually mean by ‘weight’.
And by ‘physicists’ I don’t even need to talk about NASA mission controllers. Any student in the UK taking maths or physics at A-level (high school, age 17-18) uses weight in that manner.

This whole discussion about weight and scales and whatnot is a red herring.

The OP needs to communicate that:

Yes, Virginia, there is gravity in space. We are 98,000,000 miles away from the Sun but the Earth is affected by its gravity; that’s why we orbit it instead of flying off through the galaxy. The Moon is nearly 240,000 miles away but its gravity causes the tides on Earth. So gravity has a long reach.

The sense of weightlessness in an orbiting spacecraft is due to perpetual free-fall, as has been explained. This does not mean that you are not subject to the force of gravity, just that gravity is pulling you and your spaceship at the same rate, so you feel weightless relative to the spacecraft. Such weightlessness is often simluated for training purposes in airplanes that dive, like NASA’s Vomit Comet.

However, as **UncleRojelio **and Mark Twain said, it will just waste your time and annoy them.

No it is not.

You have mass in zero gravity. You don’t have weight. For all intents and purposes “g” is zero in your space station frame of reference.
Basically the problem sounds like your coworker doesn’t understand how gravity works.

Tell him that all objects have mass and the more mass they have the more gravity they have pulling objects towards each other. Also tell him that gravity gets weaker the further away those objects are.

Next, take a bucket full of water on a rope and swing it around your head (being carefull not to hit anyone…well…at least not yet). The string is gravity pulling the astronaut (the bucket of water) towards the Earth (you) while the astronaut’s motion around the Earth tries to propell him away from it.

When those two forces are equal, the astronaut is in a free-fall orbit around the planet. He feels weightless because everything around him is falling with him at the same time and any reference points like the Earth are so far away to seem like they are almost standing still. In reality he is actually moving thousands of miles an hour.

And if he still doesn’t get it, crack him in the head with the bucket.

I presume you were originally responding to the bit of Chessic Sense’s post where (s)he said “more specifically, weight is the force exerted on an object by gravity”. I contend that that is the idea of weight normally used by physicists, at least those using Newtonian mechanics in a roughly inertial reference frame. Of course if we start talking about non-inertial reference frames, or bringing Einstein into the discussion, a more pedantic definition of weight may apply. But as Chronos implied, that is really not warranted in a water-cooler discussion.

I note that the wiki article on “weight” gives the definition you seem to favour, whereas the wiki article on “apparent weight” starts out by giving the definition of weight that I favour. So does the article on “weightlessness”.

ETA: and it would appear that your idea of “weight” is actually “apparent weight”.

Weight is force, and the SI unit is the Newton. You measure force between two objects. It isn’t intrinsic to a single thing. The earth, you, a rock, does not have an intrinsic property called weight. You measure force between two things, so in the case in question, the force exerted by a chair on the space station and you (and by Newton 1 by you on the chair) might be indistinguishable from zero, but at the same time the force exerted on you by the Earth is still about 980 Newtons. Given that the space station is in low Earth orbit, there is only a tiny change from the force felt from when you are standing on the Earth’s surface. If you weight yourself you place a force measuring device between yourself and the object you wish to be weighed relative to. Usually that other thing is the Earth.

You feel weightless when you no longer feel the force of gravity on you. Accelerating at the same rate as the force of Earth’s gravity accelerates you will do that, which can be accomplished by falling off something - until you hit the ground or air drag slows your acceleration. So getting into an airplane that does the work for you leaves you weightless. Or, as has been described above, you fall forever in orbit.

Is too. You haven’t made an argument.

Weight is the force exerted by gravity on a mass, regardless of whether anything is pushing back on it. If you jump out of an airplane you still weigh the same as you did sitting inside the airplane, and what you will weigh when you reach the ground. To say you have no weight during the fall is nonsensical. This is the scientific definition of weight. It is that weight which causes you to accelerate towards the ground. If you were weightless you would just hang in the air.

The layman’s sense of weight as being “the pressure I exert on the ground” is not scientific.

The OP’s co-worker says that there is no gravity in space, which is just ridiculous, and the point we should help address.