# Homework Help: Need info on how do helicopter stay on the same place with res to earth

1. Aug 10, 2006

### jainabhs

Hi all
I am Abhishek Jain, recently joined this group.
I have one query which might have been previously asked/explained, but I still have problems in understanding it.

I assume that the earth is spinning clockwise at 1000 miles per hour.
Suppose a helicopter is travelling anticlockwise to the spin of the earth from place A to a place called B.
In between, the helicopter stops and stays right there in the air.
Now will the place B arrive to the point where helicopter is standing in the air?

Thanks in anticipation

Abhishek Jain

2. Aug 10, 2006

### Office_Shredder

Staff Emeritus
Keep in mind that the air is moving so it's stationary to the earth also (you don't get blown away by 1,000 mph winds, do you?)

3. Aug 10, 2006

though there still might be a kinetic difference between the two...

4. Aug 10, 2006

### chroot

Staff Emeritus
If your helicopter could withstand 1,000 mph winds for the duration of its trip, this would be possible. Of course, this would still require a great deal of energy to accomplish.

In fact, think about a supersonic jet fighter flying west to east at 1,000 mph (airspeed). When viewed from, say, a distant satellite, the jet would appear to be "sitting still" in the air while the Earth rotated under it. Of course, it's really expending thousands of gallons of fuel to push itself against the drag of the air at 1,000 mph.

- Warren

5. Aug 10, 2006

### jainabhs

Hi,
Okay so I agree to your views , that the helicopter would feel air-drag and if it sustains this drag in order to stay at one point, the point B would come under.
Now this gives birth to a new confusion: If my helicopter is on point A on the earth. I take it straight up (at right angle) in the air right above point A and stay there.
Will the point B come under after certain amount of time? I think it will not.
But in answer to my first question we have agreed to that point B would come under.
Can any one tell me what is the different between these two cases?

Thanks in anticipation

6. Aug 11, 2006

### chroot

Staff Emeritus
If the helicopter goes straight up and remains stationary with respect to the air, which is itself stationary with respect to the ground, then the helicopter will remain stationary with respect to the ground. As the Earth rotates, so the helicopter moves with it.

If the helicopter endeavors to remain stationary with, say, the stars overhead, then it will have to endure a 1,000 mph wind to accomplish it.

- Warren

7. Aug 14, 2006

### jainabhs

I think following is the general explanation PLease correct me if wrong

There are basically two cases
I) First is the perfectly closed frame,like air-plane or closed wagon of train.
Here if a bee is flying , no matter in what direction it flies, it would never be hit by the aiplane or the train wagon.
Because it is a closed frame. Everything inside happens with ref to it. I think this case is clear without any doubts

II) Second case is what we were discussing, the earth and the atomosphere surrounded, stuck to earth due to gravity and decreasingly dense as the height from ground
increases.
See below two cases
a) If earth didnt have any atomosphere, there will be vaccum.
In this case spinning earth and vaccum become two different frames, and so if you fly with helicopter , you need to accelerate in in the direction os spin in order to stay on the same point, in the same way, if you stay where you are the earth will move ahead of you.
b) Now the earth has the atomosphere (As it really has)
In this case the system becomes a bit complex like, it is difficult to draw a line where atomosphere ends and vaccum starts.
Consider a case when you are on the point A with your helicopter stable on it. Now according to me the results will depend on how much above you are from that place.
If you are close to earth surface , you will be dragged along and will have inherent velocity of earth (just like case of airplane we discussed above).At this height evenif
you travel with your helicopter back and forth and then stop, still you will remain right there where you stopped.

As you elevate more you need to accelerate your helicopter in order to stay on the same place. The reason is apperant : you gained velocity V when you took off from
the earth and so that is your inherent velocity. Now as you go up, to remain on the same place you need to cover more distance than the point on the earth does.
Because now there is a significant diff in your and that point's radius of the orbit. So in order to cover more dist in the same time you need to accelerate your helicopter
then only you will stay over the same place. same is true if you are travelling back and forth.

8. Aug 14, 2006

### DaveC426913

Well, remember that, with no atmosphere, you have nothing to fight. Once you have reached your speed, you don't need to expend energy to move in that direction.

9. Aug 14, 2006

### DaveC426913

Well... the height you can attain while staying in the atmosphere is negligible in terms of affecting the distance you need to cover. The atmo is only 100 miles high at best. It might add about 0.1% to your distance needed to travel.

Yyyyeah... See now you're talking about orbits, which has nothing to do with helicopters. You now have to find some other way of holding yourself up, since there's no air. The way we do it is by going so fast we reach orbital speed. If you wanted to maintain your position over the Earth you'd have to get to 35,000 miles high!

These two things, atmosperic flight and orbital mechanics are completely unrelated, and I think you're just going to get confused if you link them.

10. Aug 16, 2006

### jainabhs

Thank you very much dave for your great feedback.