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I got this micro helicopter to play with, you know that cheap double bladed ones you can find for $25 or so. Anyway last night I was on bed trying to sleep and I got this idea, what if we put the helicopter inside a box and then seal the box, then we hover the helicopter inside the box. While the helicopter is hovering what will happen if we move the box left and right or up and down? will the helicopter move with the box without me controlling it so it's position will be relative to the box? or will it keep the position relative to the room and the box will move until it hits the blades? and why?

Sorry if this question is not about physics, physics was never my favorite and please if this is answerable here try to provide a "for dummies" answer :)

UPDATE: I just remembered something, when we are in a plane, we move with the plane! so logic says the helicopter will move with the box, correct?

enter image description here

Please excuse my MS Paint skills.

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Can someone help me with the tags, I couldn't find any tag that sounds familiar to me (like boxes or helicopters, lol) so I chose nuclear physics while I know it is not correct. –  user23971 May 4 '13 at 3:18
Thanks @BrandonEnright for the tags fix. –  user23971 May 4 '13 at 3:23
@007 Thanks Mr. Bond, but I think you are wrong, why do we move with the a plane when we are inside it then???? –  user23971 May 4 '13 at 3:33
This is essentially the same question as a helicopter or bird hovering in an elevator: physics.stackexchange.com/questions/9526 –  Brandon Enright May 4 '13 at 3:47
you are right @BrandonEnright –  user23971 May 4 '13 at 3:53
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marked as duplicate by Waffle's Crazy Peanut, Ben Crowell, akhmeteli, Manishearth May 5 '13 at 14:05

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2 Answers

up vote 3 down vote accepted

Objects don't accelerate because they're inside other objects. Objects accelerate because other objects make forces on them. The chain of cause and effect here is that the box can affect the air, then the air can affect the helicopter.

The answer to the question depends on how rapidly the box is accelerated.

To pick an extreme case, suppose that the box is accelerated so rapidly that the wall hits the helicopter at greater than the speed of sound. Until the wall hits it, the helicopter will not have felt any "disturbance in the force" from the air, because any disturbance in the air can only travel at the speed of sound.

At the opposite extreme, let the box be anchored to the earth's surface. The earth's surface is accelerating due to the earth's rotation. Clearly the helicopter won't collide with the walls when the acceleration is this small.

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I am talking about me moving the box with my hands, few meters to any direction? –  user23971 May 4 '13 at 3:45
@HaLaBi the distance doesn't matter. The peak acceleration on your movement does. –  Brandon Enright May 4 '13 at 3:48
ok thanks ...... –  user23971 May 4 '13 at 3:50
+1 for "box affects air, air affects helicopter". (In fact, if we're being pedantic, even in your "extreme case" the supersonic shockwave will hit the helicopter slightly before the wall does.) –  Ilmari Karonen May 4 '13 at 12:36
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You move with the plane because kinetically you are part of the plane's body. When you sit or walk in the plane gravity attaches you to the floor/body. you are going with the same velocity as the plane as everything in it. If you throw a ball in the air it will have the velocity of the plane already so no effect will be seen. We see effects in the plane when there are accelerations and decellerations:things in the air then will not be acquiring the extra velocity of the acceleration and will accordingly demonstrate the difference . This is because to first order air at atmospheric pressure tranmits very little energy, that is why we walk through air.

Now to the original question, a hovering helicopter in a box can only be influenced by forces at the limits, as @BenCrowell explains, so it will hit the wall when the box is moved.

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You're mischaracterizing my answer. I said the result would depend on the acceleration of the box. –  Ben Crowell May 4 '13 at 4:25
The helicopter is not independent of the air - it is suspended in it. (Pilots care about that, needless to say.) If the air accelerates upward (say) the air will soon have an upward velocity, creating more lift, causing the helicopter to accelerate upward after a time lag. Same goes for acceleration in any direction, but you're right that if the box is small the 'copter will probably hit the wall in that time lag. –  Mike Dunlavey May 4 '13 at 12:59
@MikeDunlavey yes, but the buoyancy of air is negligible, as will also be the momentum transfer to the air from the walls, for normal velocities attainable by boxes.The air will move around the helicopter and give a small push, it cannot carry it with it kinetically. –  anna v May 4 '13 at 15:53
It depends on the size of the box, whether it is a 1m cube or a 10km cube. The hovering helicopter will try to have zero velocity w.r.t the air mass it is in, not perfectly of course, but over time. This is easy to see if an updraft hits it. (I'm sure you know it has nothing to do with bouyancy - i.e. floation :-) –  Mike Dunlavey May 4 '13 at 18:43
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