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When a bird is flying or when a copter is hovering or a plane is flying what force is making them rotate with the earth beneath them?

Phiteros
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Muhammad Usman Qureshi
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They aren't. To first order, airborne objects are moving with the Earth because they were moving when on the ground. However, what they retain is their angular speed, and since the circumference increases with altitude, an object that doesn't accelerate appropriately will appear from the ground to fall behind to the West (the Earth is rotating to the East). This is a well-known phenomenon whose magnitude is latitude-dependent, falling to zero at the poles.

Carl Witthoft
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  • so if even I accelerate I will only be staying at the same spot on the ground as I go further up in the atmosphere? – Muhammad Usman Qureshi Oct 10 '16 at 13:57
  • @MuhammadUsmanQureshi If you accelerate such that you match the angular speed of the earth's rotation then you'll remain over the launch spot. – Carl Witthoft Oct 10 '16 at 14:29
  • so if I have to go in the opposite direction that is east to west I just have to remain at o acceleration , since im not accelerating ill be going westward???? meaning im going at a constant speed towards east but I end up in a place west of my current position???????? that cant be true man – Muhammad Usman Qureshi Oct 10 '16 at 14:38
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When something is flying through the air, it keeps moving along with the Earth because it was already moving that way to begin with. Remember that according to Newton's First Law, things will keep moving in the same direction unless an outside force acts on them. So when a bird, plane, or helicopter is on the ground, they are rotating along with the Earth. In the rotating reference frame of the Earth, they are stationary. From an external frame which sees the Earth as rotating, the object will be rotating in the same way. Now, when a plane takes off, it is changing its motion relative to the ground. In that reference frame, it begins moving. However, from our external reference frame, it was already moving. So its just changing its already non-zero velocity.

Phiteros
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  • That frame of reference thing i understand. But its not about take off, let me put it another way. Im in mid air in a helicopter and im just hovering 200m above the ground, going neither east nor west nor south nor north. Yet when i look down i cant see the earth rotate. Is it because gravity is pulling me with the rotation? – Muhammad Usman Qureshi Oct 09 '16 at 19:12
  • @MuhammadUsmanQureshi no it's not gravity cause gravity doesn't pull you sideways. Your momentum is already moving in that direction and the Earth's atmosphere moves largely with the Earth's rotation so it would take a great deal of work (acceleration or deceleration as you prefer) to slow down to the point where you saw the Earth rotating below you and you'd have to fight a great deal of wind resistance. Everything is already moving with the Earth's rotation, so it takes a lot of energy and work to "stop" rotating with it. – userLTK Oct 09 '16 at 20:05
  • If my momentum is moving with earth's rotation then if i want to go in the other direction i will also need a lot of force to move even a single mile in that direction if im not mistaken? But that is not the case so why is that? – Muhammad Usman Qureshi Oct 10 '16 at 04:40
  • No, because the entire Earth is moving in that direction. So from an external reference frame, it will look like you just slow down a little and allow the Earth to rotate underneath you. – Phiteros Oct 10 '16 at 04:43
  • The earth rotates from west to east right, so tell me if im wrong, but if we talk about momentum while in the air and i want to go 10 miles to the west of my current position it will take alot more force as compared to going 10 miles east. But that is not the case, why? – Muhammad Usman Qureshi Oct 10 '16 at 04:43
  • The "medium" of air is moving with the floating object and with the surface of the Earth. (for the most part, there are some prevailing winds that are slightly different than surface rotation on average). Because the relative velocity is relative to the Air, which is similar to the surface, moving 10 miles East or 10 miles West is very similar. The Earth's rotation has very little effect. Now if the Earth was rotating at close to escape velocity (a bit over 10 times faster), then you might detect a slight difference moving East vs. moving West cause you'd be close to escape velocity. – userLTK Oct 10 '16 at 05:49
  • I get that the atmosphere surrounding the planet is rotating with the earth, but with the air molecules going west to east wouldn't there be friction going against those trying to go west and help those who are going with those molecules towards east? cause speed of rotation is what more than 1k miles/h. theoretically shouldn't this be true at such a high speed? I mean at large distances like if u want to go to rio from Siberia? – Muhammad Usman Qureshi Oct 10 '16 at 14:01
  • Hi @MuhammadUsmanQureshi. Think about the oceans. They entirely rotate with the Earth - right? They do not in the slightest "fall backwards" to the west - am I right? The atmosphere is just the same. It's no big deal, really. – Fattie Oct 11 '16 at 18:55
  • Or look at it this way. Say you are just standing there. Do you feel a force pushing you Westwards? No. so if you were suddenly one inch off the ground, would you suddenly feel a force pulling you westwards? No. – Fattie Oct 11 '16 at 18:57
  • For the second time im not talking about the frame of reference, im not outside the planet. Im on the planet earth in mid air. Let me ask again, as i take off the ground i carry my momentum of the rotation of earth but i dont feel that thats for someone observing from outside thats fine. Also the whole atmosphere is moving cause of such a big rock is rotating, there fore i still dont feel the earth rotate under me. Now if i have to go opposite to the rotation, wouldn't the air molecules hinder my movement as opposed to helping me going with the rotation? – Muhammad Usman Qureshi Oct 12 '16 at 17:06
  • @joe blow, the case with the oceans is different. But when im in mid air in a helicopter im still with the atmosphere, but infact we are both moving with the rotation so when i want to go frwd thats fine if i want to go westward the atmospheric molecules should produce friction theoretically cause im no laonger in touch with ground? Am i right sri – Muhammad Usman Qureshi Oct 12 '16 at 17:09
  • Cause when im on the ground gravity is pulling me with it – Muhammad Usman Qureshi Oct 12 '16 at 17:10
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    @MuhammadUsmanQureshi The case of the ocean and the atmosphere is the same. The atmosphere is rotating along with the Earth, at the exact same rate. So if you are standing on the Earth, you are at rest relative to the air around you. Even if you jump off the ground or fly into the air, the air is still not moving relative to you. So it doesn't really help or hinder you, no matter which way you move. – Phiteros Oct 12 '16 at 17:22
  • @phirores i get that the air around me is still relative to me. But when im in mid air the movement of air molecules should have some effect if i want to go against there movement cause im no more in touch with the ground? On the ground the gravity helps me so dont feel anything going either way but once in the air gravity wont help me with that – Muhammad Usman Qureshi Oct 12 '16 at 17:27
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    Think about it this way. Imagine you're on a train that's moving. Another train pulls up alongside you and matches your speed. You then step from your train across to the other train. When this happens, you don't feel any kind of force, and you don't have to fight against the train's motion, because you're moving at the same rate as it. It's the exact same principle. – Phiteros Oct 12 '16 at 17:29
  • This really has absolutely no connection to astronomy. It's just a basic misunderstanding about relative motion. – Fattie Oct 12 '16 at 18:24
  • "Cause when im on the ground gravity is pulling me with it" that's not correct. Imagine a large chunk of the earth (like a mountain). Is the Earth "pulling it along"? Not at all. It is already in motion and just keeps going. Same with "your body". Imagine a large section of the Earth (about 1/4 of it). Is the rest of the Earth "pulling it along"? Not at all - it is very simply "in motion" and *just keeps going. No difference than if you shoot a gun out in space, the bullet will just keep going* forever. – Fattie Oct 12 '16 at 18:27
  • Consider the Earth going around the Sun. Do you think the Sun (or something else) is "pulling it around"? Nothing is "pulling it around". It just keeps going. This is the basic concept of Newtonian physics - momentum; the most fundamental idea of physics. Cheers! – Fattie Oct 12 '16 at 18:28