# An artificial satellite revolves around the earth in the circular orbit without using any fuel. But an airplane requires fuel to fly at a certain height, why?

When an artificial satellite revolves around the earth in a circular orbit, the direction of the satellite is at an angle of $90°$ with the direction of the centripetal force. So, the work done for this purpose is, $W=FS\cos90°=0$

This means that no work is required to be done, and hence, no energy has to be spent. That’s why, an artificial satellite revolves around the earth without using any fuel.

But in order that an airplane flies at a certain height, it has to overcome the force of gravity and air resistance. Also, the direction of the force and motion is not $90°$. So work should be done by the airplane and hence, energy has to be spent. This work is done by the fuels.