Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Aug 3, 2010 #1
    Hello everyone,


    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 space ship by the centrifugal force?They are not wearing belts right, do they I don't know? Thanks :smile:
  2. jcsd
  3. Aug 3, 2010 #2
    The reason they dont feel a centrifugal force is because the force thats pulling the space ship towards the force is the exact same thats 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 space ship never has to "push" them along though the turn as the car does. So they never feel the centrifugal force.

    Atleast thats what i think.
  4. Aug 3, 2010 #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.
  5. Aug 3, 2010 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    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.

    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.
  6. Aug 3, 2010 #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 doesnt mean not real. The centrifugal force can kill you, its very real.
  7. Aug 3, 2010 #6


    User Avatar
    Homework Helper

    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 space ship 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 space ship exerting a reactive centrifugal force onto the string. A wiki article covers the string and ball case showing the forces involved at various points:

  8. Aug 3, 2010 #7

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    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.

    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.
  9. Aug 3, 2010 #8


    User Avatar
    Homework Helper

    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.

    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.

    Which I thought I covered in this part of my post:

    Last edited: Aug 3, 2010
  10. Aug 3, 2010 #9
    This is not a question...
    "Why do astronauts feel weightless inside a space ship 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 space ship as they must be subject to a centrifugal force?
  11. Aug 3, 2010 #10
    Thanks for all the replies. They were very helpful :smile: I understand the answer to my question now.
  12. Aug 4, 2010 #11
    They're always falling, but since they're going so fast and aren't aiming right, they always miss the target (Earth).
  13. Aug 4, 2010 #12
    Have you ever seen one of these


    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!!
  14. Aug 4, 2010 #13

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    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.

    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.
  15. Aug 4, 2010 #14


    User Avatar
    Homework Helper

    Which is why I updated my prior post to include visual sensing of a rate of acceleration:

    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).
    Last edited: Aug 4, 2010
  16. Aug 4, 2010 #15

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    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.

    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.
  17. Aug 4, 2010 #16


    User Avatar
    Homework Helper

    Wiki defines fictitious force as an apparent force, which is how I thought it was defined:


    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook