# What forces carry hovering objects around the earth with its rotation?

My questions are:
a) What significant forces “carry” flying objects around with the rotation of the earth,
b) How do each of those forces contribute to that “carrying”, and
c) How relatively significant is each of those forces?
d) If any of the 5 forces below are not significant contributors, please state why not.

For example, if an aircraft was to take off, thrust directly upwards and hover at say 30,000 feet for 12 hours, then come directly down again, I think we'd generally agree that it would land in the same city it took off from, not half way around the earth. The forces we have considered are:

• Gravity
• Centrifugal force
• Centripetal force
• Momentum (or is it inertia?) of the object before it left earth
• The atmosphere's rotation with the earth
• Any other significant contributors?

I have my opinion on which of the above are relevant, but I didn’t do much Physics at university (30 years ago), and I’d like to know what you think and why. because a friend and I disagree.

• Yes, @yatisagade, that was the StackExchange link I was alluding to in my original post (the paragraph that Danu has edited out for us), (and I've seen others), but it doesn't answer everything I've asked, including things like: If gravity is not significant, then why is it not?. I think I understand the physics involved, but my friend and I disagree on the answers, so I am trying to get a better understanding by asking people who should know. My friend thinks he knows the answer, but can't recall the details of the reason, and hasn't got the time to get back into it. – Terry May 31 '15 at 10:34
• Possible duplicates: physics.stackexchange.com/q/1193/2451 , physics.stackexchange.com/q/58154/2451 and links therein. – Qmechanic May 6 '19 at 13:42

You don't need a complicated answer. The answer is the fact that we are moving too.

How can this bird swoop down and catch the worm if the ground and the worm are rotating so quickly? The answer is because the ground, the air, and the tree are all moving at the same rate. The same applies to flying objects. So the forces involved are the same forces that keep everything else rotating: mainly momentum and gravity (momentum is inertia in motion).

We should remember (or learn now) that momentum only works in a straight line. Gravity forces your flying object to follow the same course that it would on earth.

The green arrows are gravity. The blue lines are momentum. They add to form a curved line. The same line you are following, the bird is following, and the worm is following.

• A well explained answer, thank you Jimmy! Good pictures, too. Now looking at part c of my question, how would you compare the significance of gravity vs. momentum, in keeping the hovering object above the same city as the earth rotates, i.e. which one is more significant and why? I have my ideas on this (which I'd be happy to share later), but I'd like to hear yours first, please. Also, do you have a Physics degree? If so, where from? Thanks. – Terry Jun 1 '15 at 8:53
• @Terry They are both equally significant because it would not work without the both. Both are necessary to for the curved line of motion around the earth. – Jimmy360 Jun 1 '15 at 12:25
• @Terry I do not have a physics degree as of yet. – Jimmy360 Jun 1 '15 at 12:25
• Thanks Jimmy. Personally, I was thinking the momentum would be much more significant than gravity in this situation. Here’s my thoughts on why: 1. Gravity is simply stopping the craft from continuing in a straight path out into space. 2. Gravity is not contributing to the “horizontal” movement of the craft. 3. For every 1 m of “horizontal” movement (from momentum), far less than 1 mm of “downward” adjustment from that straight course (from gravity) would be required, and this could be achieved with a slight downward thrust. – Terry Jun 1 '15 at 22:53
• 4. Take away the gravity, and the principle still basically works over relatively short distances (i.e. the object only relatively-slowly gets further from earth). Imagine your bird & worm example without gravity, for example. 5. But if you retain the gravity and have no momentum (i.e. the earth spins under an object which is not moving with it), the principle would not work at all. – Terry Jun 1 '15 at 23:28