When someone jumps off a tall building why are they free from gravity until they hit the ground?
What makes you think they’re free from gravity? If they are free from gravity, why do you think they are accelerating towards the ground?
They are not. For if they were free from gravity, they would never accelerate towards the ground.
They are free of the ground though, until, as the OP notes, they hit it.
They are, of course essentially weightless, in that if they were standing on scales as they descended, no weight would be shown. Their mass would remain constant, which is what will cause the problems at ground level.
It’s easy to become weightless. Just jump into the air - at the point where you stop going up and start coming back down - you are weightless.
Because I thought it is not the person moving but the earth moving towards the person. So when something is in freefall they are really standing still but the earth is not, but I might have this wrong, which is why I am asking. I know the universe is always expanding so it seemed logical.
I saw a video yesterday where Brian Greene (the well know popularizer of physics) did a demonstration showing a container filled with water with holes in its sides which he dropped and the jets of water escaping from the holes no longer went down but up. This seemed to show that the container was no longer subject to any acceleration because it was not fixed to anything. So in freefall it seems there are no forces acting on a body, but I might be confused about this.
Thanks Bob. But what causes gravity? Is it just the acceleration of a body?
You may be remembering the other thread which hypothesized people playing baseball and jumping up and down inside a big cylinder rotating in space.
By contrast, as Isaac Newton figured out, when you have two massive objects they accelerate towards each other.
Gravity is always working. It just gets weaker the farther that you are from the mass you are falling towards.
Gravity works in both directions. So if you jump out of an airplane, you are being pulled down by the mass of the Earth. But the mass of your body is also simultaneously pulling the Earth up towards you. However because the Earth is so much more massive than your body is, the movement of the Earth towards you is negligible in comparison to the movement of you towards the Earth. But both forces result in you and the Earth coming together.
What’s probably confusing you is air resistance. Air resists having objects move through it. The amount of the resistance varies depending on the surface area of the object and its mass. So you can see two objects, like a nail and feather, falling at different speeds. It’s even possible to have enough air resistance that it is stronger than gravity and prevents the object from falling down. But the gravity is still there and is still acting on the object.
The container and the water are both in freefall. This means that any water that escapes through the sideholes (it would be significantly less in free fall than when stationary, since the water will have no weight, and therefore exert no pressure) would stay level with the container.
However, I expect the water may appear to travel upwards relative to the container because air resistance would affect individual water drops more than the container. (It would be slight, but probably noticeable).
Is this the vid: https://www.youtube.com/watch?v=0jjFjC30-4A (Starts about 4 mins in(.
No I didn’t see that thread, but is that not due to angular momentum? The kind of thing I remember seeing in* 2001: A Space Odyssey?*
Yes and I guess this must be due to the warping of each other’s space, that I do know about but then is this the answer, i.e. the geometry of spacetime? I’m getting awfully:confused: confused.
That’s a good question. We know pretty much how it works and what it does, but as far as I know, the ***cause ***of it is still a mystery.
Another way of thinking about things is this:
You are always being attracted to the Earth, the acceleration of the attraction is 9.8ms[sup]-2[/sup]. Most of the time however there is some pesky solid object in the way, and that is stopping that acceleration moving you in the direction of the gravitational attraction. So it just pushes up on your feet stopping you from moving down. If you remove the solid object under your feet, you are no longer being pushed up, and become free to move in the direction of the attraction. So you no longer feel the force on your feet pushing up, which you interpret as now being weightless. If you are able to accelerate for long enough you may reach the point where you are travelling fast enough that the air resistance starts to matter, and it starts to counter the gravitational attraction. Eventually you reach terminal velocity, and the force of the air resistance creates an acceleration that matches that of the gravity, and you cease to accelerate towards the ground any more. That air resistance is now doing the job that the ground was originally doing (and will shortly start doing again in a very abrupt manner.)
Brian Greene’s container of water is essentially doing the same. If he had done the experiment in a vacuum the water jets would have gone out horizontally - as there would be no air drag to bend them upwards (ignoring that a vacuum would have resulted in the water boiling.) But the big trick is to think carefully about the reference frame you are watching the dropping bucket in. If you have a camera attached to the bucket, you see the jets stop bending down, and go out sideways. If you watch the bucket fall in the frame of reference from which it was released you will see nothing change, the water outside of the bucket will continue to fall exactly the same as it did before - falling toward the ground with an acceleration of 9.8ms[sup]-2[/sup]. Dropping the bucket simply means that the bucket and attached camera is now keeping up with the falling water that jetted out. This is free fall. When you are in free fall you feel no external forces on you (like the ground pushing up) and so feel weightless.
One assumes there is some additional mechanism to make the water exit the bucket, when in free fall there is no force to create a head of water. Balloon of water maybe?
Ah, that’s clear enough Nemo. Thank you. But just to check my understanding: If I were to jump off a building standing on a weighing machine, although I would be in freefall there would still be a little force acting on the weighing machine because there would be a tiny bit of gravitational attraction between me and the machine. When I reached the ground (discounting the impact) the resistance of the ground would stop my fall and I would experience my full weight. Is this right?
(BTW, I knew about different falling masses being affected equally by gravity thanks.) 
That was not explained, so I much appreciate your fuller explanation. I have learnt something new. Thank you. 
Oh yeah, I don’t expect everything.
Great explanation Francis. I now know a lot more than I did. Thanks.
Well, I’ve looked at the video again and all he did was to take the cap off the water container and he said that allowed air pressure in. Anyway here is the video link: https://www.youtube.com/watch?v=HneFM-BvZj4
I’m not going to get into the cause of gravity… but should say that gravity is not an acceleration. It is a force proportional to mass. This force will either be countered by some equal and opposite force, or it will cause acceleration.
When we’re standing on a solid surface, the “down” force of gravity is countered by an upwards force of the solid surface supporting us. That creates the usual sensation of weight. When you’re falling, there’s very little force to counter gravity, and you’re accelerating downward. (Air resistance will counter a little.)
Actually, you’re weightless as soon as you jump. No matter which direction you’re going, if gravity is the only force acting on you, then you are “weightless.”
If they’re free from gravity, why do they hit the ground at all? If I were free from gravity, I’d go UP to a higher building, and step onto its roof.
Are these people stupid, or is the stress of plummetting from the top of a skyscraper distracting them from noticing the obvious solution?