# Rotation of the earth

1. Apr 29, 2010

### thereddevils

I am not sure if this is the correct subforum to ask this question . My apologies in advance .

Consider a helicopter 'floating' at a certain height from the ground and it's stationary , say if it's in London , would it still be in London after 12 hours ? Since the earth is rotating , would it still be in the same place as where it was initially ?

We are actually moving as the earth rotates because we are in contact with the ground but this time the helicopter isn't ?

2. Apr 29, 2010

### Staff: Mentor

Well, you're specifying the answer in the question: if it is stationary, it is stationary.
Ask yourself what forces might prevent a helicopter from accelerating to 1,000 mph (the speed of earth's rotation). Or conversely, what would cause it to accelerate? Remember, when it takes off, it starts from zero speed.

Last edited: Apr 29, 2010
3. Apr 29, 2010

### thereddevils

Of course its stationary , but the earth is rotating so whatever the person in the helicopter sees(surrounding) is changing though he and the helicopter is stationary .

4. Apr 29, 2010

### D H

Staff Emeritus
You and Russ are talking at cross-purposes, thereddevils. Russ is objecting to the way you described the problem. In the original post you said the helicopter is stationary. Stationary with respect to what? An observer on the ground? Tautologically, if the helicopter is stationary it is stationary. It ain't moving with respect to the ground because you already said so.

The easiest way to address such problems (easiest if you have the requisite math/physics background, that is) is to look at things from the perspective of an Earth-fixed observer. This is a rotating reference frame, so fictitious forces such as centrifugal and Coriolis forces will arise if you want to use F=ma. The helicopter is stationary, so there is no Coriolis force here. Centrifugal force at London's latitude is about 0.0027 g. That is a rather small force, smaller than that due to even a slight breeze.

5. Apr 29, 2010

### HallsofIvy

In other words, "stationary" relative to what? You seem to be thinking "stationary" means "stationary relative to the center of the earth" and Russ Watters is assuming you mean "stationary relative to the surface of the earth".

Certainly if you are talking about a helicopter that takes off, goes straight up a few hundred feet and then hovers, the second interpretation is the correct one.

Relative to the center of the earth, the helicopter sitting on the ground is already moving at about 1000 mph. In order to remain "stationary relative to the center of the earth" the helicopter would have to accelerate (or decellerate) by that much- and I don't know of any helicopter that can do that.

Last edited by a moderator: Aug 8, 2011
6. Apr 29, 2010

### Cleonis

You seem to attribute a special status to being in contact with the ground.
This raises the question: what constitutes 'being in contact with the ground'?

Let me provide a definition in terms of the opposite:
- If you are not in any way in contact with the Earth then you are in free fall.

Example: if you are standing on a trap door, and the trap door suddenly opens, then you go into free fall.
Another example of free fall is an orbiting satellite.

Going back to the trap door: as long as that trap door is up it is supporting your weight. The trap door is exerting an upward force upon you, sustaining your position.

Now to the case of a hovering helicopter.
Is the weight of the hovering helicopter supported? Indeed it is. The helicopter blades are producing lift, thus sustaining the altitude of the helicopter.

Summerizing:
The helicopter is not in free motion; it's altitude relative to the Earth is sustained by an upward force. In that sense the helicopter is in contact with the Earth, just as much as we surface dwellers are.

Last edited: Apr 29, 2010
7. Apr 30, 2010

### thereddevils

Thanks ! i understood better now . Another thought came to me ,

Consider this situation where you are in a car which is moving at a speed of 100 km/h . There is a pen suspended from the front mirror . After some time , the string holding the pen snaps , so the pen is supposed to hit you since the car is moving but it didn't (i experimented it) . Why ?

8. Apr 30, 2010

### Staff: Mentor

DH explained it more, but in any case, did you read the rest of my post...?
You really should have read the rest of my post! I pointed you toward the answer and the answer is the same in both cases. If the pen is moving at 100mph, what would possibly make it suddenly stop when the string breaks (heck, if the string is hanging straight down, it isn't even pulling the pen forward anyway!)?

This is Newton's first law: "In the absence of a net force, the center of mass of a body either is at rest or moves at a constant velocity." The pen doesn't hit you in the face because there is no force pushing it toward your face. It moves at a constant velocity because a force is required to change a velocity: no force, no change in velocity.

You can also look at the helicopter example from the other direction: if a helicopter were moving at 1000 mph with respect to the ground, how would that feel to the helicopter? 1000mph of wind in the face of the helicopter would certainly have an effect, wouldn't it? For a helicopter that flies at 120mph, how could it handle that much of a headwind?

Last edited: Apr 30, 2010
9. Apr 30, 2010

### pallidin

OK, well, why a heliocopter?
Why not just a smooth bowling ball on a sheet of ice.

Does the bowling ball move?

10. Apr 30, 2010

### thereddevils

11. Aug 8, 2011

### LeoNCW

since that the helicopter didn change it's position on earth because it were travelling in the speed of earth rotating , what if there is a UFO from the other planet and never land on earth floating on the earth ? will the situation be the same?

12. Aug 8, 2011

### Staff: Mentor

You haven't defined the scenario, so I dont know if it is the same. Either way, though, Newton's laws don't care what planet you are from.

13. Aug 8, 2011

### mathfeel

For a helicopter to be able to be stationary with respect to center of earth, it'd have to be at such speed to go around the earth in 24 hrs (as seen from ground). That doesn't seems like helicopter speed.

Helicopter works by lift, so the useful measure of speed is its relative speed with air, which is basically dragged along by earth's rotation (otherwise, it'd very windy everywhere).

14. Aug 8, 2011

### Lsos

I know it's old, but I'm not sure I agree with the above reasoning. It's not the fact that the helicopter is supported by the air that makes it "In contact with the earth", it's the fact that it IS in the air....and the air moves with the earth. Freefall or not, if the earth was spinning at 1000mph below you, it means you would be braving 1000mph winds, and that takes ALOT of effort.

A skydiver, for example, doesn't all of a sudden accelerate to 1000mph relative to the earth just because he's in free fall....