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Suppose an object comes from space and is put some kilometers away from the earth's surface (where there is low density of air).Will it rotate with the earth?

Now, suppose a helicopter lift off the earth and accelerates until imitates the behaviour of the space object (so there is now implications of the principle of inetia).

I think this idea would imply to travel faster or to use much less energy. Is that right?

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closed as unclear what you're asking by ZeroTheHero, Jon Custer, Qmechanic May 10 '17 at 2:50

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  • $\begingroup$ a hovering helicopter with no air - wtf ;) $\endgroup$ – Ilja May 9 '17 at 20:51
  • $\begingroup$ I do not know how to say it, but the problem is clear, isn't it? $\endgroup$ – santimirandarp May 9 '17 at 20:56
  • $\begingroup$ no, actually my comment was not just humorous :) the air is not moving with respect to the ground (it sometimes does, but the energy is consumed by friction pretty fast; and wind velocities are negligible compared to the earth rotation) - so there is no way to neglect air resistance if you want to move with respect to the earth. Just no way. $\endgroup$ – Ilja May 9 '17 at 21:03
  • $\begingroup$ I think density of air is very small some kilometers away $\endgroup$ – santimirandarp May 9 '17 at 21:08
  • $\begingroup$ Yes, that's why airplanes fly so high. But it has nothing to do with earth's movement. They just don't care, that earth is moving or rotating. I assume, you wanted to use an object that comes from outside the earth (it doesn't have to be a helicopter, take some asteroid, they are more common out there) and use its movement to travel with respect to earth. This might work (theoretically, not in practice). But you cannot reuse the object, it will travel on after you disembark. To get it back, you need to put in energy, which you wanted to avoid. $\endgroup$ – Ilja May 9 '17 at 21:18
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No. The helicopter will not move with respect to the ground if it is hovering. It will remain in the same spot. It is moving WITH the rotation of the earth and earth's atmosphere.

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    $\begingroup$ then, technically, after 24h it would be in its initial position :-P $\endgroup$ – AccidentalFourierTransform May 9 '17 at 19:01
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    $\begingroup$ but what is the force which move the helicopter? $\endgroup$ – santimirandarp May 9 '17 at 19:02
  • $\begingroup$ The force of friction has accelerated the helicopter (and everything on the surface of the earth for that matter) to that of the rotation of the earth. Everything we make or interact with has already been accelerated to this speed upon the formation of the planet so it is imperceptible to us. $\endgroup$ – Brad S May 9 '17 at 19:06
  • $\begingroup$ friction with the air? And why air moves with the earth? $\endgroup$ – santimirandarp May 9 '17 at 19:09
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    $\begingroup$ @santimirandarp Consider what would happen if the atmosphere did not move with the rotation of the Earth: at the equator the relative speeds would be about $460\,\mathrm{m/s}$, which is comfortably supersonic. Frictional forces from wind would rapidly dump angular momentum from the planet into the atmosphere at the surface, and turbulent mixing within the atmosphere would propagate this, until it was (approximately) co-rotating with the planet. It would be entertaining to set up an atmospheric simulation to model this: I expect they do not deal well with supersonic winds though. $\endgroup$ – tfb May 9 '17 at 19:37
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I assume, you want to use an object that comes from outside the earth (it doesn't have to be a helicopter, take some asteroid, they are more common out there) and use its movement to travel with respect to earth?

This might work (theoretically). But you cannot reuse the object, it will travel on after you disembark. To get it back, you need to put in energy, which you wanted to avoid.

If you wanted to build a helicopter for this purpose, then ask yourself: how does it get there (in front of the earth and with no initial velocity). You will need the same energy to prepare it which you save afterwards.

Well, and in practice... ;) catching asteroids is quite a hard task; it could be easier to build a great windmill working on incoming asteroids :) a funny idea, maybe the energy source of the future.

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  • $\begingroup$ Does a helicopter move with the rotation of the earth if it is some kilometers (or at a low density distance) from earth? Why? I think this is not an answer $\endgroup$ – santimirandarp May 9 '17 at 21:31
  • $\begingroup$ If it came from outside, it will move on. If it started from earth, it will remain (nearly) above the same spot. Why? Well, why not, because it initially has the velocity of rotation already. The same reason why we can remain where we are. What is the difference between sitting in front of your computer and hovering? There is no force on you when you are sitting either. $\endgroup$ – Ilja May 9 '17 at 21:37
  • $\begingroup$ If i say it came from space, it is because I understand inertia. You do not answer why this wouldn't be a good idea to spend less energy. $\endgroup$ – santimirandarp May 9 '17 at 21:41
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    $\begingroup$ If it came from space, you have to had put it there (and give it a velocity with respect to earth) before; this energy counts, too. Let's make a break for today, me at least; maybe you will see it differently too if you sleep over it :) $\endgroup$ – Ilja May 9 '17 at 22:03

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