Why do astronauts feel weightless inside a space ship and centrifugal force?

In summary, the conversation discussed the concept of centrifugal force and its role in circular motion. It was explained that centrifugal force is not a real force, but a fictitious one that is used to explain motion in a rotating frame. The conversation also touched on the use of centrifugal force in elementary physics classes and the confusion it can cause. The idea of reactive centrifugal force was also brought up, but it was clarified that this is an outdated understanding of the term. Ultimately, it was emphasized that understanding the basics of motion in an inertial frame is crucial before delving into concepts like centrifugal force.
  • #1
sameeralord
662
3
Hello everyone,

centrifugal.free.gif


Ok so I read that the reason they are in free fall is because they are going in a circle like this , and the Earth curves away from them at the same time or something, could anyone explain it to me simply. Also why don't astraunauts get pushed to the periphery of the spaceship by the centrifugal force?They are not wearing belts right, do they I don't know? Thanks :smile:
 
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  • #2
The reason they don't feel a centrifugal force is because the force that's pulling the spaceship towards the force is the exact same that's pulling them. When a car makes a sharp turn for example, you feel like getting pushed the other way, this is becase the wheels when turned are only exerting a force on the car and not you directly. When the car turns, it exerts the force on you which gets you turning with the car.

In the case of the astronauts, the force of gravity is acting on the spaceship but at the same time, its also pulling the astronauts themselves so the spaceship never has to "push" them along though the turn as the car does. So they never feel the centrifugal force.

Atleast that's what i think.
 
  • #3
In addition to what Routaran said, the engineers and scientists that plan the launch calculate the exact elevation and velocity needed so that the gravitational force and centripetal force balance each other out. Since the forces cancel each other out, the net force on the astronauts is zero (they feel weightless). The net force on the spaceship is also zero (no air resistance in space), so the spaceship will continue to orbit on its own without the use of any propulsion.
 
  • #4
The reason you don't feel centrifugal force is because it isn't a real force. It is a fictitious force.

The reason you don't feel gravity in free fall is because every single part of the body is subject to almost exactly the same gravitational force. There is no tension that will let you feel the force. You don't even feel the gravitational force when standing on the ground. What you feel is the normal force, the Earth pushing up on you, propagating through your body.

I need to say something before I proceed: sameeralord, this is not a ding at you. Please don't take it that way. This is a ding at teachers. Actually, its a rant.

<rant>
Using centrifugal force to describing orbits, or circular motion in general, is a widely used approach in many elementary physics classes, and it is wrong. Look at the picture in the OP! There is no centrifugal force in that view: The particle is moving in a circle. There is no circular motion in a frame where centrifugal force balances centripetal force. There is no motion whatsoever. The particle just hanging in one spot.

There is no reason to talk about centrifugal force until the basics of motion in an inertial frame are well understood. There is no reason to mix frames, ever. Drawings like that, and they appear in elementary texts all the time, that are mixing frames.
</rant>
 
  • #5
"The reason you don't feel centrifugal force is because it isn't a real force. It is a fictitious force."

what? You can feel a centrifugal force. It is real, fictitious doesn't mean not real. The centrifugal force can kill you, its very real.
 
  • #6
One issue here is that there is no string causing the circular motion, just gravity. If the orbit is circular, the centripetal force is the gravitational force exerted onto the spaceship in the direction of the earth, and unlike the diagram, the equal and opposing force is also a centripetal gravitational force exerted onto the Earth in the direction of the space ship. It's a two body system, that rotates about the common center of mass. If the orbit is elliptical, then depending on the time in the orbital cycle, there is a linear component of force as well as radial.

If there was no gravity involved, and instead a string used to create the circular motion, then you'd have the forces as shown on the diagram, the string exerts a centripetal force onto the spaceship, coexistant with the spaceship exerting a reactive centrifugal force onto the string. A wiki article covers the string and ball case showing the forces involved at various points:

http://en.wikipedia.org/wiki/Reactive_centrifugal_force
 
  • #7
Academic said:
what? You can feel a centrifugal force. It is real, fictitious doesn't mean not real. The centrifugal force can kill you, its very real.
Fictitious means not real. Those fictitious forces are not real in a very real sense: There is no way to sense them. If you are referring to that silly xkcd comic (and yes it is funny), that isn't a centrifugal force. It is a centripetal force.


rcgldr said:
If there was no gravity involved, and instead a string used to create the circular motion, then you'd have the forces as shown on the diagram, the string exerts a centripetal force onto the spaceship, coexistant with the spaceship exerting a reactive centrifugal force onto the string. A wiki article covers the string and ball case showing the forces involved at various points:

http://en.wikipedia.org/wiki/Reactive_centrifugal_force
That is an ancient and very misleading meaning of the term "centrifugal force" that physicists have been trying to stamp out for half a century or more. It is misleading for two reasons. Firstly, overloading definitions in science is a bad idea. The standard meaning of the term centrifugal force is that of a fictitious force that is needed to explain motion in a rotating frame while pretending that Newton's laws still apply. Secondly, it's only centrifugal (away from the center) when you look at the wrong center.

Think of it this way: Imagine a binary star system in which the two stars are more or less of the same mass. The two stars are orbiting the center of mass of the two stars thanks to the gravitational force the two stars exert on one the other. The force star A exerts on star B is directed toward star A, and the force exerted by star B on star A is directed toward star B. These are central forces. Each is in a very real sense a centripetal force. Neither can be viewed in any reasonable way as a centrifugal force.

The 3rd law reaction to a centripetal force is a centripetal force, not the terribly named reactive centrifugal force.
 
  • #8
D H said:
Fictitious means not real. Those fictitious forces are not real in a very real sense: There is no way to sense them.
In a rotating frame of reference, a fictitious force can be sensed, such as standing on a scale to measure weight while in an rotating space station (or noting the rate of acceleration in what appears to be free fall). What is actually an inertial force appears to be a real force from the rotating (or any accelerating) frame of reference.

rcgldr said:
reactive centrifugal force
D H said:
That is an ancient and very misleading meaning of the term "centrifugal force" that physicists have been trying to stamp out for half a century or more. Overloading definitions in science is a bad idea.
Then why did physicists decide to change the meaning of an existing (ancient) term instead of creating a new one? The argument here is with several English dictionaries and the wiki article. I only cited that wiki article because it included a diagram and terminology that corresponded to the OP.

D H said:
binary star system ... the 3rd law reaction to a centripetal force is a centripetal force, not the terribly named reactive centrifugal force.
Which I thought I covered in this part of my post:

rcgldr said:
If the orbit is circular, the centripetal force is the gravitational force exerted onto the spaceship in the direction of the earth, and unlike the diagram, the equal and opposing force is also a centripetal gravitational force exerted onto the Earth in the direction of the space ship.
 
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  • #9
This is not a question...
"Why do astronauts feel weightless inside a spaceship and centrifugal force?"

Before it can be answered we need to better define the question. Do you mean.. Why do astronauts feel weightless inside a spaceship as they must be subject to a centrifugal force?
 
  • #10
Thanks for all the replies. They were very helpful :smile: I understand the answer to my question now.
 
  • #11
sameeralord said:
...could anyone explain it to me simply...?...

They're always falling, but since they're going so fast and aren't aiming right, they always miss the target (Earth).
 
  • #12
Have you ever seen one of these

cominorbiter.jpg


you put a coin in the slot and it rolls around the funnel and spirals down. When your first insert a coin, it barely gets closer to the center. That's kind of how gravity works. Forget all that stuff about centrifugal forces and centripital forces. The coin (aka astronaut) wants to fall straight toward the center, but the horizontal movement is so great that it takes a very, very long time for it to get any closer to the center (earth). You'll notice that when you put a coin in one of these machines, and the coin starts to slow down (due to friction) it falls towards the center faster.

Also, if you hold two objects out in space, but close enough to be effected by gravity, they will fall straight down, but will still curve towards each other!
 
  • #13
rcgldr said:
In a rotating frame of reference, a fictitious force can be sensed, such as standing on a scale to measure weight while in an rotating space station (or noting the rate of acceleration in what appears to be free fall). What is actually an inertial force appears to be a real force from the rotating (or any accelerating) frame of reference.
The scale in that rotating space station is measuring a real force, not a fictitious one. It is measuring the normal force, which is an electromagnetic phenomenon. The direction of that force is toward, not away from, the axis of rotation. It is a centripetal force.

To an inertial observer, the person standing on the scale is subject to that one force, and nothing else. That one force makes the person undergo circular motion. To an observer fixed with respect to the rotating space station, the person standing on the scale is not moving. Yet the scale is registering a non-zero force, pointing inward. The fictitious centrifugal force (directed outward) arises from insisting that Newton's laws apply in this frame. The net force on the person must be zero since the person on the scale is not moving. We know there is one force present; it is the force being registered by the scale. There must be some other force, directed outward, that is counterbalancing this inward force. That outward force is the fictitious centrifugal force.

This force cannot be sensed by a scale or by any other local experiment. No fictitious force can. They are a figment of the observer's imagination -- but they are very useful at times. Accurately modeling the Earth's atmosphere is essential for forecasting hurricanes and other foul weather. Doing that from the perspective of an Earth-fixed frame (a rotating frame) requires the use of fictitious forces. This is a tough but doable task. Modeling the Earth's atmosphere from the perspective of an inertial frame is a fool's errand.

Then why did physicists decide to change the meaning of an existing (ancient) term instead of creating a new one? The argument here is with several English dictionaries and the wiki article.
The sense of a fictitious force is the older meaning. It certainly meant that to Newton when he explained why gravity is apparently reduced at the equator. The full development of the physics of non-inertial frames came 100 years or more after Newton. Somewhere along the line that reactive centrifugal force nonsense was added to the mix.

Most physics texts do not teach that nonsense. Dictionaries are not scientific references, and neither is wikipedia. That particular wiki article exemplifies why wikipedia should not be used as a scientific reference.
 
  • #14
D H said:
The scale in that rotating space station is measuring a real force, not a fictitious one. It is measuring the normal force...
Which is why I updated my prior post to include visual sensing of a rate of acceleration:

rcgldr said:
In a rotating frame of reference, a fictitious force can be sensed ... (or noting the rate of acceleration in what appears to be free fall).

D H said:
fictitious force is the older meaning. It certainly meant that to Newton when he explained why gravity is apparently reduced at the equator.
But in Netownian (versus general relativity) physics, gravity is a real force, and to an observer in a rotating (or otherwise accelerating) frame of reference the fictitious centrifufgal force (or fictitious linear force if a linear accelerating frame of reference) appears to be as real as gravity, observed by the rate of acceleration during "free fall".

As mentioned before I posted the wiki link because it included a similar diagram and terminology as the OP and explained that. I also pointed out that since gravity and not a string was involved here, that both Earth and spaceship experience a centripetal force due to gravity. I forgot to include that both are in "free fall" (if you ignore external forces like gravity from the sun).
 
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  • #15
rcgldr said:
Which is why I updated my prior post to include visual sensing of a rate of acceleration:
Visual sensing of acceleration is not a local experiment. A scale is a local experiment.

Besides, all that visual sensing of acceleration tells you is that a body-based frame is not an inertial frame and you should not be using Newton's laws.

rcgldr said:
D H said:
The sense of a fictitious force is the older meaning. It certainly meant that to Newton when he explained why gravity is apparently reduced at the equator. The full development of the physics of non-inertial frames came 100 years or more after Newton. Somewhere along the line that reactive centrifugal force nonsense was added to the mix.
But in Netownian (versus general relativity) physics, gravity is a real force, and to an observer in a rotating (or otherwise accelerating) frame of reference the fictitious centrifufgal force (or fictitious linear force if a linear accelerating frame of reference) appears to be as real as gravity, observed by the rate of acceleration during "free fall".
Ummm? Where did this come from?

I was talking about Newton, not Einstein. I intentionally have not raised the issue that gravity itself is a fictitious force in GR; this is an introductory physics question, so bringing up GR is not a good idea.
 
  • #16
D H said:
fictitious force.
Wiki defines fictitious force as an apparent force, which is how I thought it was defined:

http://en.wikipedia.org/wiki/Fictitious_force

I was just trying to help out with an explanation of the OP's diagram, not get into a debate over terminology. In the other thread about a marble in a glass tube, I avoided using the term "centrifugal" for this very reason.
 

1. What causes astronauts to feel weightless inside a space ship?

Astronauts feel weightless inside a space ship because they are in a state of constant freefall. The space ship is orbiting the Earth at a high speed, causing the astronauts and the ship to constantly fall towards the Earth, but they never actually reach the surface. This creates the sensation of weightlessness.

2. How does centrifugal force play a role in this weightlessness?

Centrifugal force is not actually responsible for the weightlessness experienced by astronauts. It is a perceived force that occurs when an object is moving in a curved path. In the case of a space ship, the centrifugal force is equal to the force of gravity, canceling it out and creating the feeling of weightlessness.

3. Does the size of the space ship affect the weightlessness experienced by astronauts?

The size of the space ship does not affect the weightlessness experienced by astronauts. As long as the space ship is in a state of freefall, the sensation of weightlessness will be present regardless of its size. However, a larger space ship may have more room for the astronauts to move around and perform tasks while weightless.

4. Can astronauts experience different levels of weightlessness inside a space ship?

Yes, astronauts can experience different levels of weightlessness inside a space ship depending on its altitude and speed. The closer a space ship is to Earth, the stronger the force of gravity will be, resulting in a slightly less weightless feeling. Additionally, if the space ship is accelerating or decelerating, the level of weightlessness may change.

5. Do astronauts feel weightless in all parts of the space ship?

Yes, astronauts feel weightless in all parts of the space ship as long as it is in a state of freefall. This includes the cockpit, living quarters, and even the bathroom. However, if the space ship is accelerating or decelerating, the force of gravity may be slightly stronger in certain areas of the ship.

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