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I was reading in a book about the basic concept of launching satellites into orbit. I came across a paragraph saying,

If we take a body high above the surface of the earth, and then project it with some horizontal velocity, it will describe a parabolic path and fall back onto the Earth's surface. If the magnitude of the velocity is increased, the horizontal range also increases. We keep on increasing the horizontal velocity of projection, till at certain point, a stage will come when the body will not hit the earth, but will always be in a state of free fall under gravity in an attempt to fall to the earth.

I did not exactly understand the last statement .What do they mean by always be in a state of free fall ?

This may seem basic but I'm having trouble visualising it. If someone would explain what is the meaning of the last statement, that would help me a lot.


EDIT: After reading the answers, I have one last doubt. How would we analyse the motion of the satellite then ? Would it be rectilinear motion, like any other free falling body, or circular motion around the earth, as we generally think of a satellite, or both( I don't understand how that would work). Or does it completely depend on the frame of reference? Thanks for your time.

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The satellite will always be falling towards the Earth. The trick to achieving orbit to have enough tangential (horizontal) velocity to constantly 'miss' the Earth. To be in a state of free fall means that the only force acting on you is gravity. This is true in this case, since there is no friction, drag, etc in space.

So if you are constantly falling towards the Earth, you also need to move very fast tangentially, to always avoid crashing. This is true for satellites, the space station, and even the Moon!

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  • $\begingroup$ @koldrakan-Could you just have a look at my question again, please. I have made an edit. Thanks for your answer by the way. Helped me understand it better. $\endgroup$ – devb Apr 20 '17 at 12:08
  • $\begingroup$ @Nightshade - I don't fully understand the question in your edit; Are you asking how we would describe the motion of an object in orbit? $\endgroup$ – koldrakan Apr 20 '17 at 14:02
  • $\begingroup$ Yes... But that would be parabolic, right ? But we're also saying its in free fall. So it's in free fall but describing a parabolic path around the earth ? $\endgroup$ – devb Apr 20 '17 at 14:46
  • $\begingroup$ @Nightshade it follows an elliptical path and is in freefall. It goes forward and is being pulled towards the Earth at the same time, and the result is an ellipse $\endgroup$ – koldrakan Apr 20 '17 at 19:34
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The paragraph is correct but a bit ambiguous. Imagine a wall of height 'h' amidst a fountain or lake of certain radius. If now you increase the horizontal velocity such that it crosses the fountain, then your job is done. Replace the fountain or lake with earth and cliff with that particular height where they project the satellite from. Nevertheless the satellite will always be in a state of free fall.

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  • $\begingroup$ Nice example. Also, is there any particular term for that certain velocity of projection? Or is that the final orbital velocity itself ? $\endgroup$ – devb Apr 20 '17 at 12:10
  • $\begingroup$ @Nightshade I am not aware of it. But the velocity that allows any object to escape earth's gravitational field is called escape velocity. If my answer helped you atleast click on tick. $\endgroup$ – Shivam Kumar Apr 20 '17 at 13:03
  • $\begingroup$ @Nightshade Regarding the analysis if motion ; the motion closely follows elliptical path. $\endgroup$ – Shivam Kumar Apr 20 '17 at 13:06
  • $\begingroup$ Could you answer the question I have added as an edit to my original question ? I will accept your answer after that, no doubt. $\endgroup$ – devb Apr 20 '17 at 13:23
  • $\begingroup$ Sorry didn't see your second comment. Thanks $\endgroup$ – devb Apr 20 '17 at 13:42

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