# How do Satellites Stay Up?

Satellites orbit the earth because of the force of gravity. To understand why this happens and why the satellite does not get pulled in and fall, we have to understand what forces do. A force will change the motion of an object; it might speed it up, slow it down or change its direction. For example, if you are running and someone pushes you from behind, you speed up (the force is in the direction of your motion). But if someone pushes you in the chest when you are running, you slow down (the force is in the opposite direction to your motion). If you are running and someone pushes you from the side, you move away from them, changing your direction. (the force is at right angles to the motion). This idea is called

Newton’s First Law.

To make something move in a circle it must be moving and have a force that is always at right angles to the motion so that it constantly changes direction. This force is called the centripetal force.

Imagine swinging a rock on a string around your head. The tension in the string pulls the rock round in a circle (this is the force at right angles to the motion). So the tension is the centripetal force. If we cut the string, the rock will continue in a straight line because there is no longer a force to change its direction. For a satellite, the centripetal force is the gravitational force, the pull of the earth. If we could switch gravity off, we would lose all our satellites as they move off in straight lines!

Going back to the rock example, we need to put energy into keeping the rock moving because the rock is moving through air and is losing energy constantly because of air resistance. We don’t need to do this with satellites because they are moving through space where there is no air, so no air resistance acts on the satellites and they don’t slow down.

Many people think there is a centrifugal force acting which pulls the satellite (or rock) outwards. This is not the case; there is no such thing as a centrifugal force. Imagine riding in the back seat of a car as it turns the corner. Let us assume the back seat is very slippery and you don’t have a seatbelt on. As the car turns the corner, you slide from the inside to the outside. You could argue that a force is pushing you outwards. In fact the reason you move outwards is that there is no force keeping you moving in the same circle as the car. What you really do is continue in a straight line. Eventually you will hit the far door and the door then pushes you providing the centripetal force to keep you moving in the same circle as the car. If the car door was open you would not have a centripetal force acting and you would continue in a straight line out of the car! (Don’t try this at home!)