# Fan in vacuum

by amey_naik2812
Tags: vacuum
 P: 8 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????
 P: 772 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?
 PF Gold P: 7,120 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?
 P: 8 Fan in vacuum 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????
 PF Gold P: 7,120 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.
 P: 8 @jack.... its just a question with popped in my head.... but my intuition says it will move... dnt know why..
 P: 8 @pengwuino but my question is will the fan move or remain at the same position???
 P: 772 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?
 P: 8 wont the rotation of the blades of the fan create a force which might push the fan backwards... newtons law TO EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION...
 P: 38 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? = )
 P: 772 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."
P: 23,727
 Quote by texasblitzem I think the fan blades will spin and the base will slowly start to spin in the same direction as the blades
Same or opposite?
PF Gold
P: 7,120
 Quote by amey_naik2812 wont the rotation of the blades of the fan create a force which might push the fan backwards... newtons law TO EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION...
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.
 P: 8 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??
P: 772
 Quote by amey_naik2812 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??
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.