# Having a problem with Centrifugal/Centripetal force in zero gravity

1. Mar 20, 2013

### fivetide

Hi I’m having a problem with a concept, well actually with a force. After watching a program about sending man to the other end of the galaxy in which a huge tube was created in where people lived and gravity was simulated by spinning it and producing Centripetal force. My problem is that I don't believe Centripetal is a single force, but more a tertiary force of gravity. In other words I thought physically that for Centripetal force to work you need gravity. I might not be making myself clear but if you just imagine a cylinder spinning in space there is no physical anchor to its inside. Basically if you lift your legs up or jump for instance other than a slight bit of air turbulence the cylinder floor would simply rotate under you because there is no connection via gravity or otherwise between you and the cylinder. I understand that they use centrifugal force to adjust the international space station; I can understand this working, as a space station has a certain amount of attraction to the Earth in other words sharing its gravity. If this was not true the space station would simply sit there while the Earth carries on its elliptical tour of the sun, and then eventually the Earth would catch up with it and collide. The only way I can see it working is, if the cylinder itself had a mass great enough for it to attract the mass of a person, it wouldn't have to be a huge amount but I don't think that would be feasible. There is no difference between the zero gravity on the outside of the cylinder and the inside of the cylinder. If you accelerate someone in space to say 10,000 miles an hour you would experience the force generated by the acceleration not gravity, as soon as the ship has reached its terminal velocity zero gravity does not simply return it was simple always there. As far as I can see generating gravity using Centripetal force is total science-fiction, am I wrong?

2. Mar 20, 2013

### stevendaryl

Staff Emeritus
I'm not sure I understand your concern, but the "artificial gravity" that is achievable by spinning is something that you can test yourself: Take a bucket, and put some water in it, and then spin the bucket in a circle. The water will be pressed to the bottom of the bucket, and will not spill out (if you're spinning it hard enough).

Some amusement parks have a ride where you get into a cylindrical room, with your back against the wall of the cylinder. Then the cylinder starts spinning, and you are pressed against the wall.

So this effect is not science fiction, it's definitely real.

3. Mar 20, 2013

### fivetide

Yes I do understand that , but that depends on gravity being present, same scenario but in zero gravity, if you spin the bucket with water in it the water would simply float away into space and all you would have is a spinning bucket. Nothing is attracting the water to the bucket. On earth the water is being pulled down towards it , causing pressure, the pressure forces the water to make contact with the sides of the bucket the energy generated by the spinning is transferred by it proximity to the water, the water then starts to spin itself. Also if you start to spin water it takes a while for the whole of the water to start spinning.
The people at the amusement arcade are being initially pinned to the floor of the cylinder by gravity and then the centre of mass is changed by the spinning of the disk they are standing on , one force pinning them to the floor the other forcing them against the cylinder.

Last edited: Mar 20, 2013
4. Mar 20, 2013

### Staff: Mentor

No, the centripetal force involved when spinning something has nothing to do with gravity. (Although there are plenty of situations where gravity contributes to the centripetal force.)

5. Mar 20, 2013

### fivetide

I still believe, in zero gravity, that there has to be a physical attraction between the cylinder wall and the object and that nothing would gravitate towards the walls just because it’s spinning.

6. Mar 20, 2013

### Crazymechanic

You may believe in what you want , the question is do these things believe in your believing... [Puts on FBI type sunglasses ,The Who - we won't get fooled again starts to play in the background]

Gravity is a property of mass/energy , it simply attracts it does not care whether you move or not.
Now the centripetal force is different it works only when movement is present it works only when there is something that either rotates or changes direction like a car making a corner , as you know gravity pushes the car down not sideways so there is other force that makes it want to continue going in a straight line.
Now when you spin something in a circle or a round tube now lets say there is a ball inside of it the faster you spin it the higher the speed of the ball and the more pressure it exerts on the walls of the circle because it constantly tries to get out if to to travel in a straight line but it cant because it is kept locked and spun around.
Now in space there might be many possibilities a space craft can rotate with no net movement in any direction and it can rotate with also a movement in some direction according to an observer that sees it from aside.
You too can stand and spin the bucket and you can walk while spinning the bucket but the bucket doesn't care whether you walk or stand the pressure of the water inside remains the same.
The earth has gravity and moon does too but they also have an opposite force acting against it and that is the centripetal one because just as earth attracts moon moon also has a tangential velocity which makes it push in the opposite direction just like a car in a corner and that is a counter force to gravity so it cannot be gravity r gravity dependent.
Now this is when the velocity is constant , acceleration makes it change.

7. Mar 20, 2013

### Staff: Mentor

Well, you're wrong in one sense, right in another. If you are floating in a cylindrical space station and all of a sudden the walls began to rotate, nothing much would happen. But grab onto the walls so that you are rotating along with them and you'll find yourself pushed down against the cylinder walls. Once you're rotating along with the cylinder, you won't have to hold on anymore--you'll be pushed down. That's why it's useful for creating an artificial gravity.

8. Mar 20, 2013

### stevendaryl

Staff Emeritus
You don't need anything attracting the water to the bucket. What you do need is a way to get the water in the bucket spinning. So, for example, if you put a lid on the bucket, then spin it, the water will be forced to the bottom of the bucket. At that point, you can take the lid off, and the water will stay in the bottom as long as you keep spinning the bucket.

You're certainly right that if there is nothing to keep the water in the bucket initially, then it will just float away.

So, yes, if you have giant cylindrical space station that is spinning, then it is possible to float forever at the center of the cylinder. You won't be pushed to the walls of the cylinder. However, if you do move in the direction of one of the walls, you will eventually hit the wall. At that point, friction will soon get you moving at the same speed as the walls. When you are moving at the same speed as the walls, you will "feel" an apparent gravitational force pushing you against the wall.

9. Mar 20, 2013

### fivetide

In which direction is the force pushing you down, I know that sounds like I have answered my own question, but you are saying you will forced against the wall of the cylinder, but the essentially the cylinder is spinning away towards you depending on which direction you are looking. So what you are saying is that if once you are spinning with the cylinder you jump up you will automatically fall back towards it? Or will the cylinder spin underneath you? Where is the downward force coming from ? At some point your speed and the speed of the cylinder will equalise, two equal forces cancel each other out, a LaGrange point, you would just be back in Zero gravity, actually you never left but that’s not my point.

10. Mar 20, 2013

### stevendaryl

Staff Emeritus
Gravity has nothing to do with the force that pins you to the wall of the rotating room. It's purely a function of your velocity. If you are traveling at the same speed as the wall, then you will experience a force pressing you into the wall (or perhaps I should say, you will experience a force of the wall pressing up on you).

11. Mar 20, 2013

### fivetide

stevendaryl You don't need anything attracting the water to the bucket. What you do need is a way to get the water in the bucket spinning. So, for example, if you put a lid on the bucket, then spin it, the water will be forced to the bottom of the bucket. At that point, you can take the lid off, and the water will stay in the bottom as long as you keep spinning the bucket.

If you take a bucket of water into absolute zero gravity than the first thing the water would do is form a sphere, what you are saying is if you compress the water into the bucket then the water under pressure will stick to its side. I agree you have a volume of liquid under pressure forced inside the container, however that doesn't translate to a human in air standing on the wall of the cylinder. There is no force or pressure pushing back human against the cylinder wall.

You're certainly right that if there is nothing to keep the water in the bucket initially, then it will just float away.

So, yes, if you have giant cylindrical space station that is spinning, then it is possible to float forever at the center of the cylinder. You won't be pushed to the walls of the cylinder. However, if you do move in the direction of one of the walls, you will eventually hit the wall. At that point, friction will soon get you moving at the same speed as the walls. When you are moving at the same speed as the walls, you will "feel" an apparent gravitational force pushing you against the wall.

I think that in my last post I think that once your speeds have equalised you won't feel anything at all there is absolutely no force attaching you to the wall of the spinning cylinder.

12. Mar 20, 2013

### fivetide

I didn't say that it did what I said was that the gravity initially pins you to the floor and as the floor starts to rotate gravity both holding it down and at the same time the base spinning will force you to the side. Take away the base standing on the grass with the cylinder rotating around you walk up to the wall of the cylinder and let me know if you attach to it, because the initial contact caused by friction would spin your body around and then you would fall onto the grass.

13. Mar 20, 2013

### stevendaryl

Staff Emeritus
Yes, if you are inside a big rotating cylinder, pressed against the wall, and jump up toward the center, you will fall back down, provided that you don't jump too high.

Think about it this way: If you are in a rotating cylinder, pressed against the wall, then your velocity is tangential to the wall. If you continued moving in the same direction, you would break through the wall. So to keep you from breaking the wall, the wall has to push up on you. So to keep you pressed against the wall, the wall has to exert a radially inward force on you. That's what you feel when you feel artificial gravity. What you feel when you are standing on the wall is the force of the wall pushing up against your feet.

The same is true of real gravity. Standing on a floor, you don't feel gravity, you feel the force of the floor pushing up on you. If the floor stopped pushing up on you (by breaking), you would immediately stop feeling gravity. The same is true of the artificial gravity inside a rotating cylinder. You don't actually feel the artificial gravity, you feel the force of the cylinder wall pushing up on you.

14. Mar 20, 2013

### stevendaryl

Staff Emeritus
Gravity has nothing to do with it. The only thing that is relevant is that as the room starts to spin, you have to spin with it. If you hold onto the wall, that will be enough to get you spinning. You don't need gravity, and it has nothing to do with the effect.

I don't know what you are talking about. The only relevant thing for the effect is that your velocity has to be synched with the velocity of the wall that you are touching. That can happen by friction, or by holding onto a bar connected to the side, or whatever.

15. Mar 20, 2013

### Crazymechanic

If the floor breaks you don't stop feeling gravity you start feeling pain while lying with a broken arm on your neighbors table, yet gravity is still there.

Now as for the OP a rotating chamber or whatnot is not gravity and it does t need gravity it works on the principle that objects that travel with a certain speed in a circle tend to always push on the outer side because they want to travel away in a straight line but they cant.This is the tangential velocity.

Put a metal ball a tiny one in a paper , wrap it around the ball and attach it to a string then turn it around increasing the speed the weight of the ball pushing against the side of the paper will also increase as the kinetic energy of the ball will get higher from it's higher tangential velocity and eventually the ball should brake through the paper.

16. Mar 20, 2013

### stevendaryl

Staff Emeritus
Well, this isn't a matter of opinion. It's an effect that is well understood theoretically, and is well demonstrated experimentally. What you're saying is wrong, both theoretically and experimentally.

If you are inside a cylinder, and you have a nonzero velocity, then you will eventually collide with the wall of the cylinder. At that point, friction will rapidly get you moving at the same speed as the wall. As long as you have a nonzero velocity, you will "fall" to the wall.

17. Mar 20, 2013

### stevendaryl

Staff Emeritus
I'm not saying that gravity isn't there, I'm saying that you don't feel it. Jump off a cliff, and close your eyes. While you are falling, you don't feel any gravity. You feel air whistling past you. When you hit the ground, you feel the ground smashing into you. But there is never a point where you feel any gravity.

18. Mar 20, 2013

### Crazymechanic

Maybe but you do feel the acceleration that gravity makes on you.While in the OP's original sense he speaks about a theoretical place with no gravity.In such a place you don't fall you don't feel anything but if you get in a rotating chamber then you can feel the pressure against the wall of the chamber which is identical to that of gravity only the cause is different , now did we nailed it ? :D

19. Mar 20, 2013

### Janus

Staff Emeritus
When you are "standing" on the inside surface of the cylinder you share the tangential velocity of that part of the cylinder. If you jump up, you still keep that velocity. for someone not spinning with the cylinder you will travel in a straight line at a velocity due to the vector addition of your jump velocity and the tangential velocity. This straight line intersects the cylinder wall at two places, one being the point at which you jumped. The other will be some distance in the direction the cylinder is spinning.

Since the cylinder is spinning, the physical spot from which you jumped will be rotating into a new position while you are in the air. In fact, it will move to almost exactly the same point where your straight line trajectory will intersect the cylinder. To you, it appears as if you jumped straight up and came down back in nearly the same spot. (how close you come to landing in the same spot depends on the radius of the cylinder and how hard you jump.)

20. Mar 20, 2013

### Staff: Mentor

The cylinder walls (and you) are accelerating towards the axis of the cylinder.
Since you are in a rotating reference frame, inertial forces are introduced. If you drop a ball, it will act as if a force pulled it toward the wall. This is described as being a centrifugal force, which is an artifact of being in a non-inertial, rotating reference frame.
Huh? Looked at from an inertial frame, you are accelerating towards the center of the cylinder; it is the cylinder that provides that centripetal force. Viewed from the rotating frame, which would be quite natural if you were the one in the cylinder, you are not accelerating so the centripetal force (the upward push from the wall) and the centrifugal force would balance out. Nothing to do with zero gravity or LaGrange points.