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Fan in vacuum

  1. Oct 22, 2010 #1
    if a fan with a battery is placed in space outside earth and if it is switched on.....
    then what will happen????
    will the fan go ahead, behind or will it remain at the same position????
    Last edited: Oct 22, 2010
  2. jcsd
  3. Oct 22, 2010 #2
    I'm curious as to what your intuition tells you. What do you think will happen, and why?

    Also, what do you think makes propellers move on Earth?
  4. Oct 22, 2010 #3


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    What makes a fan spin? If just put some fan blades on a pole in still air, do you think they'd move? Why do fans need electricity?
  5. Oct 22, 2010 #4
    i agree that the fan wont move on the pole, but if u switch it on then wind will be generated....
    wat do u say????
  6. Oct 22, 2010 #5


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    Sure, if there is air around the fan, the fan will push the air around.

    If you're in space where there is no air, the fan will spin, but there's no air to be pushed around.
  7. Oct 22, 2010 #6
    @jack.... its just a question with popped in my head....
    but my intuition says it will move...
    dnt know why..
  8. Oct 22, 2010 #7
    but my question is will the fan move or remain at the same position???
  9. Oct 22, 2010 #8
    Things only move if there is a force acting on it. What force would be acting on the fan in space? What force acts on the fan on Earth?
  10. Oct 22, 2010 #9
    wont the rotation of the blades of the fan create a force which might push the fan backwards...
  11. Oct 22, 2010 #10
    I'd like to chime in on this if no one minds. I think the fan blades will spin and the base will slowly start to spin in the same direction as the blades, just like a helicopter would if it did not have the tail rotor to counteract the rotation of the main rotor. But now I have a question, if the above is true.
    Say the battery lasts forever, would the base start to catch up in speed to the fan blades? It seems the blades always have to go faster than the base, or else you'd end up with a spinning object in space. How fast would the base spin relative to the blades?
    Or is my post nonsense? = )
  12. Oct 22, 2010 #11
    The rotation of the blades would pull the center of the fan along in the direction of motion.

    However, the blades go around in a circle, and we're assuming a symmetrical fan. For example, when the top blade is moving right, the bottom blade is moving left. When the right blade is moving down, the left blade is moving up. NONE of the blades is moving along the axis of rotation (which is what you mean by forward).

    The forces associated with the fan blades moving cancel out.

    Now, if you had an unbalanced fan, with, say, three blades on one side and none on the other, the fan would probably wobble around a little, but still wouldn't "go forward."
  13. Oct 22, 2010 #12


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    Same or opposite?
  14. Oct 22, 2010 #13


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    Yes but there is no air to push against so there's no force in the first place so the "equal and opposite" force wouldn't exist either.

    However, as someone has pointed out, if the fan blades rotate, the rest of the fan will rotate as well, in the opposite direction to conserve angular momentum. So if you started up the fan in space, the fans would start going lets say, counter-clockwise, and the base of the fan would start turning clockwise. However there would be no movement of the system as a whole.
  15. Oct 23, 2010 #14
    i would like to give an example....
    take a torpedo and a missile into consideration, which are to move and hit a target underwater. Both work on the principle of action and reaction. Missile uses the force generated my compressed air while the torpedo has a fan at the back to push it forward...
    correct me if i m wrong.
    and if the space craft which burns the fuel to propel itself forward then why cant fan move or if the fan is attached to an object then wont the object move?????
    its just that the medium has changed for torpedo and missile the medium was water but for the space craft and the fan the medium is vacuum....
    now the argument is that vacuum is nothing so there is nothing to push against and if that is the case then how come space ships move according to the principle of action and reaction??
  16. Oct 23, 2010 #15
    Rocket boosters in space don't work by pushing air out of the way. It works by "equal and opposite reaction."

    Propellers (or fans) work by pushing air of the way. There is no air in space to push out of the way.

    Surely you can see that rockets are actually firing particles out of the back end, while a fan in a vacuum fires out no particles.
  17. Oct 23, 2010 #16
    jack please elaborate:)
  18. Oct 23, 2010 #17
    Look at a rocket. You see particles flying out of the back. They propel the rocket forward even in a vacuum.

    Look at a fan in a vacuum. There are no particles!

    I don't see how I can make it any more clear.
  19. Oct 23, 2010 #18


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    As Jack said, a missiles propulsion is coming from a controlled explosion. You have a chemical reaction that sends molecules flying away from each in opposite directions. Half the molecules in general fly out one end. The other end fly into the rocket, providing a push in a sense (don't quote me on that being exactly what it does but the basic idea is that the propulsion is due to reactions of chemicals already on the rocket). This is with no input coming from the atmosphere surrounding the rocket (except the oxidizer on a solid rocket i believe?). In fact, being in an atmosphere, if I recall, actually reduces the effectiveness of rockets a little, though again don't quote me on that.

    A torpedo needs to push on something. It doesn't have its own supply of molecules to shoot out the opposite direction of where it wants to go. A rocket does.

    Which makes me wonder if solid rockets work in space considering I believe they use oxygen in the atmosphere as an oxidizer.

    EDIT: Ok so as I suspected, the solid rockets propellant has an oxidizer in it. Thus, the atmosphere a solid rocket is in is irrelevant, air or vacuum. Thank you Google.
    Last edited: Oct 23, 2010
  20. Oct 23, 2010 #19
    Hello All,

    A fan in a vacuum would behave as such...

    When turned on, the motor inside the fan will receive energy from the battery, which in turn, will spin the propeller.

    However, being a vacuum, there is no medium in which the propellers would be pushing against.

    Propulsion from a propeller of, lets say, an aircraft, works by rotating through the air like a corkscrew or a screw through wood.

    In space the propeller on our fan would spin but would have nothing to interact with.

    The base of the fan would also be affected. For every force is an equal and opposite reaction.

    If the motor is mounted to the base at a 90 degree angle, as most conventional fans, the amount of force that is needed to rotate the propellers in a clockwise direction is applied in the opposite direction where the motor is mounted.

    The base would spin in the opposite direction as the blades.

    Rocket propulsion works in a vacuum because the exploding force on the bottom of the rocket creates a force in the opposite direction. Keep in mind rocket fuel has a mixture of fuel and oxidizer (oxygen).
  21. Oct 23, 2010 #20
    thank you everyone :)
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