Can you create gravity in space? Also a question on space debris

[flyingpig]

Homework Statement

The following questions are all out of curiosity.

1. Let's say I have an object like maybe a cup sitting in space. If I let it stay there, will it move under some constant velocity such that at some time it will have a momentum equal to a bullet?

2. If you are in a spaceship with zero gravity and you are just floating in the spaceship. If suddenly the pilot decides to rotate the spaceship (with respect to the floor of the spaceship) 360, will that create an instantaneous G-field - in other words everything falls to the ground? I saw this in a space-sci-fi show lol. I couldn't draw a FBD on it, is it the infamous "fictitious force" that they call G-field?

 

[cepheid]

Homework Statement

The following questions are all out of curiosity.

1. Let's say I have an object like maybe a cup sitting in space. If I let it stay there, will it move under some constant velocity such that at some time it will have a momentum equal to a bullet?

I'm afraid I don't quite understand. Can you clarify what you are asking? You say the cup is "sitting" in space. Is that meant to indicate that it is at rest? If so, how could it somehow have a momentum equal to a bullet at some later time? Are there forces acting on the cup? If so, its velocity would not be constant.

Of course, all motion is relative, so even if we're considering a particular reference frame in which the cup is at rest, we can always construct another reference frame (moving at significant speed relative to the first one) in which the cup does have the momentum of a typical bullet.

2. If you are in a spaceship with zero gravity and you are just floating in the spaceship. If suddenly the pilot decides to rotate the spaceship (with respect to the floor of the spaceship) 360, will that create an instantaneous G-field - in other words everything falls to the ground? I saw this in a space-sci-fi show lol. I couldn't draw a FBD on it, is it the infamous "fictitious force" that they call G-field?

Objects floating freely in the middle of the spacecraft would be unaffected by the rotation. Objects that come into contact with the walls may be accelerated.

A continuously rotating ring-shaped object has been proposed (and used in sci-fi many times) as a way of generating "artificial gravity" in space. Objects in contact with the outer ring experience a centripetal force, and if the spin rate is just right, this could be equal to Earth gravity, allowing you to "walk" around the outside wall.

 

[flyingpig]

I'm afraid I don't quite understand. Can you clarify what you are asking? You say the cup is "sitting" in space. Is that meant to indicate that it is at rest? If so, how could it somehow have a momentum equal to a bullet at some later time? Are there forces acting on the cup? If so, its velocity would not be constant.

No, I mean just in space. It was on a sci-fi show. It was taken in the future where journey in space became something common. It was about a Space Debris group, basically taking out "space trash" like nails, screws, planks, and outdated satellites. Somehow all of them were traveling at 8km/s (not sure how even though they were at rest).

Objects floating freely in the middle of the spacecraft would be unaffected by the rotation. Objects that come into contact with the walls may be accelerated.

A continuously rotating ring-shaped object has been proposed (and used in sci-fi many times) as a way of generating "artificial gravity" in space. Objects in contact with the outer ring experience a centripetal force, and if the spin rate is just right, this could be equal to Earth gravity, allowing you to "walk" around the outside wall.

So it must be in the shape of a "ring"? Why does it fail when the shape is not?

 

[cepheid]

No, I mean just in space. It was on a sci-fi show. It was taken in the future where journey in space became something common. It was about a Space Debris group, basically taking out "space trash" like nails, screws, planks, and outdated satellites. Somehow all of them were traveling at 8km/s (not sure how even though they were at rest).

Ah, see now that's different. I thought you meant a cup just in space somewhere completely isolated. Space debris is in orbit around Earth. These objects are next to a planet, so of course there is a force acting on them. Can you figure out what the orbital speed of an object in a circular orbit 100 km above the surface of the Earth must be?

Remember also that terms like "at rest" are all relative. The space garbage team would probably match speeds with a given piece of debris in order to make it easy to retrieve it.

So it must be in the shape of a "ring"? Why does it fail when the shape is not?

No, sorry. It doesn't have to be in the shape of a ring. That just seems kind of convenient when it comes to having a continuous surface to walk along.

 

[ghostanime2001]

For the second question. Can you create a gravitational field inside the spaceship like Earth's surface gravitational field? If that were possible wouldn't aeronautic engineers find out by now and then calibrate it to approximately the same magnitude and we would all be space men/women already XDDDD

 

[cepheid]

For the second question. Can you create a gravitational field inside the spaceship like Earth's surface gravitational field? If that were possible wouldn't aeronautic engineers find out by now and then calibrate it to approximately the same magnitude and we would all be space men/women already XDDDD

You can't really create a gravitational field, but you can simulate the effects of gravity in the manner I described. I think that to reach a force equivalent to Earth gravity, you'd either have to have the spaceship rotating really fast (which is uncomfortable for other reasons) or make it really large, which would be more suited to a space station. This has been proposed, but it represents a highly costly and technically challenging endeavour. Just look at how much went into building the ISS.

In any case, I don't think weightlessness is the biggest hindrance to space flight becoming commonplace. There are other reasons why that hasn't already happened.

 

[ghostanime2001]

Is it the worry that human bones become weakened while in space? or medical reasons? Not that I am hinting health is the only obstacle preventing humans from inhabiting the space station or other celestial planets.

 

[flyingpig]

Ah, see now that's different. I thought you meant a cup just in space somewhere completely isolated. Space debris is in orbit around Earth. These objects are next to a planet, so of course there is a force acting on them. Can you figure out what the orbital speed of an object in a circular orbit 100 km above the surface of the Earth must be?

So nothing is really at rest if you are close to the Earth's orbital?

No, sorry. It doesn't have to be in the shape of a ring. That just seems kind of convenient when it comes to having a continuous surface to walk along.

What would FBD look like? I can't draw it out. Can you draw fictitious forces on FBD? It's not really a temporarily G-field right? They said "G-field" which is physically wrong right?

 

[cepheid]

So nothing is really at rest if you are close to the Earth's orbital?

Well I have a sort of two-part answer to your question. The first part is sort of an indirect answer. Before we continue with any sort of discussion about motion, I feel the need to point out that there is no absolute notion of what is "at rest" and what is "moving." All motion is relative. What I mean by that is, one observer could claim that a certain object was at rest, while another observer (who is in motion relative to the first one) could equally well claim that the same object was moving. It all depends on your reference frame. Providing that both observers are in inertial reference frames, then their statements about the motion of the object are equally valid. Formally, an inertial reference frame is one in which Newton's laws are obeyed. Practically speaking, what that means it that inertial reference frames move at a constant velocity relative to each other. So, just to drive the point home: if Observer A and Observer B have a constant velocity relative to each other, then considering the situation "from Observer A's reference frame" is equivalent to considering Observer A to be stationary, in which case Observer B is moving. HOWEVER Observer B can claim with equal validity that he is the one who is at rest, and that it is Observer A who is moving.

The above discussion doesn't hold true for accelerated reference frames. Accelerating reference frames are non-inertial (meaning that Newton's laws do not hold within them). Typically, in order for an observer to be able to explain the motions that he/she sees while in an accelerating reference frame, and still have things make sense in the framework of Newton's laws, that observer must introduce so-called "fictitious forces." For example, if you're in a car that's going around a curve, you're in an accelerating reference frame. If you still want to "pretend" that you are stationary, you're going to have to introduce a fictitious force in order to explain what you experience. In this case, out of nowhere, there seems to be a "centrifugal" force that pushes on you in the direction towards the outside of the turn.

Now that we've clarified those points, here is a more direct answer to your question above: yes, all objects near Earth experience its gravitational pull. Therefore, these objects have a force acting on them and are accelerating.

By the way, you didn't answer my question! What is the orbital speed of an object in a circular orbit 100 km above the surface of the Earth? Here's a hint: the object is moving in a circle, therefore it must have a centripetal force acting on it. In this case, the centripetal force is provided by Earth's gravity (which, after all, pulls towards the centre of the orbit). That last statement gives you all the information you need to work out the object's speed, and if you do, you'll find that whoever wrote your sci-fi series came up with quite a plausible value for the orbital speed of the space junk.

What would FBD look like? I can't draw it out. Can you draw fictitious forces on FBD? It's not really a temporarily G-field right? They said "G-field" which is physically wrong right?

I don't know what a "G-field" is, that's not really a proper physics term, and it is probably something the sci-fi writers made up. But, to answer your question: you are right. There is NO actual gravity involved here. Rotating the space station merely allows the effects of gravity to be simulated. From a FBD point of view, the situation is similar to above. A person in contact with the outer wall of the rotating space structure is in circular motion, and therefore he has a centripetal force acting on him. Due to his inertia, at any instant that person wants to "fly out" of the circle in the tangential direction. But the inward-pointing centripetal force prevents him from doing so. In this case, the centripetal force is due to the normal force exerted by the wall on the person. He feels a normal force pushing upward on his feet in just the same way that a person on Earth (in a gravitational field) feels such a normal force. If you get the station spinning fast enough, you can get it so that that normal force is equal to the typical force that a person feels pushing up on his feet when he is on Earth (i.e. mg). See the attached figure.

EDIT: I should point out that science fiction writers seldom use this plausible way of simulating the effects of gravity in space, because it is too restrictive from a story-telling standpoint (esp for TV). They often just invent a futuristic (and conveniently unexplained) new technology that can somehow generate "artificial gravity" (just like in Star Trek, for example). So, I wouldn't spend too much time scratching my head over the "physics" used in this sci-fi show you were watching if I were you. Scientific accuracy is not exactly the writers' top priority.