Ask a Physicist Answers

Why does a stationary body above the surface of the earth not change its position after some time while the earth is moving? Suppose if we place an object above the surface of the earth, would it be at the same position after some time and why?

The earth rotates from west to east once around in 24 hours. Since the earth’s circumference at the equator is about 25,000 miles, the surface speed at the equator is something more than 1000 mph. Away from the equator the surface speed decreases, dropping to zero at the poles. Things on the surface of the earth and the air above the surface have the same speed from west to east. Now consider a hot air balloon on the surface of the earth on a day when the air is perfectly still. Perfectly still means that the air has the same speed from west to east as the earth’s surface, but it is at rest relative to the surface. As the balloon is inflated with hot air it will eventually begin to float and rise into the air. As it rises it still has the same speed from west to east and so remains stationary relative to the earth’s surface. If the hot air is adjusted so that the balloon doesn’t rise or sink, it will remain stationary as long as the air remains still. No force is needed to prevent the balloon from moving horizontally. It will continue moving with same speed from west to east and will remain stationary relative to the earth unless some force causes it to speed up or slow down.

Answered By: Kenneth S. Mendelson, Marquette University