QUESTION #203

# If you hover any distance above the earth (in a helicopter, for example) for a day or so, why aren't you and the helicopter occupants on the other side of the world if the Earth is going through its normal revolution?

I am glad you asked this question because it gives all of us a chance to think about things we take for granted we know until some one asks about it and then we realize we don't know what we thought we did. We discover that something that was obvious is not so clearly understood.

When you hover in a helicopter you are hovering in air. This air is moving along with the earth. In other words, the earth and the air above it are moving around and around on the earth's axis together.

Do you find this hard to believe? Many people do and so many people wonder, like you did, why this helicopter would not circle the earth by standing still for one day. But, this is why your question is so good. It is the kind of question that makes one stop and think of things that we normally do not think about and when we do we find ourselves surprised.

So, let's think about it for a second. What would it be like if the air did not circle the earth with us? Do you have any idea how fast the earth is spinning on its axis? The earth is about 6 million meters in radius. So its circumference is 2r which is about 6 million times two times which comes to about 38 million meters around the equator. The earth travels this distance in one day; so that's 38 million meters divided by 24 hours giving us about one and a half million meters per hour which comes to about a thousand miles per hour. A hurricane can have winds of about 150 miles per hour and that causes a lot of damage. Can you imagine a constant wind of 1,000 miles per hour? Wow! Some storm!

So, you can see that it is a good thing that the air we live in moves around the earth with us. If you place a helicopter in that air, or a leaf, or a balloon, it will move with the air and not the earth!
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

Yours is the same argument made long ago by proponents of the 'stationary Earth, moving Sun' philosophy. 'If the Earth is moving,' they asked, 'shouldn't the ground move beneath my feet when I jump straight up, causing me to land elsewhere as the Earth moves along without me?'

You can see for yourself, on a smaller scale, how this works as you travel in a car or airplane at a constant velocity. Toss a ball straight up and it will drop straight back down relative to you, NOT falling behind as you move on ahead. The ball maintains its initial forward MOMENTUM in addition to its up and down motion. An observer outside your vehicle would see the ball travel along a parabolic path, combining its horizontal with its vertical motion.

Newton's first law says that an object in uniform motion will continue that motion as long as it is not acted upon by an outside force. It the case of a jumping person, a thrown ball, or a helicopter, the initial horizontal motion (or angular motion on the larger scale of a round Earth) that any object has when it leaves the Earth's surface stays with it even after contact with the surface is lost. That, by the way, is why satellites are launched to the east and near the equator, where the Earth's velocity provides the most 'free' momentum before the rocket even gets off the ground.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor