# Why doesn’t the earth collapse on the sun?

A player hits a golf ball. The ball eventually comes to rest due to air friction and bouncing on the court.

To an Earthling, that seems like a logical question. After all, we live in a world where everything eventually comes to a standstill. A car with engine trouble stops on the hard shoulder, a knocked-out golf ball can be found somewhere (hopefully) on the course and a merry-go-round will stop if you don’t continuously pump energy into it.

That seems quite normal, because everything on Earth experiences friction. The car stops due to headwind and the contact of the tires with the road surface. A golf ball loses its speed after the launch due to air friction and bouncing on the course. The merry-go-round stops due to the friction in the shaft on which the installation rotates.

Friction is the process of converting kinetic energy into heat. In all these examples, the kinetic energy has been released as heat to the atmosphere, the road surface, the tires, the golf course or turning axis.

### Exceptional situation

From a more cosmic perspective, this is quite an exceptional situation. If you go to space, many forms of friction disappear. In the absence of an atmosphere, there is usually no (significant) headwind and a spaceship usually does not make contact with a surface that creates friction. That means a spaceship with bad luck not comes to a stop on the interplanetary hard shoulder. It simply keeps flying straight ahead at the same speed! The first scientist to write this down was Isaac Newton. He formulated the famous law of conservation of energy.

The movement of a planet around a star (or for example a moon around a planet) is a bit more subtle. Without the sun, the earth would (continue to) fly straight ahead. The gravity of the sun bends the orbit of the earth to a circle around the sun, but our planet loses no kinetic energy in the process. Basically, a planet will keep spinning for free until the end. Incidentally, there are subtle friction and tidal effects that influence the speed and trajectory of celestial bodies.

From a cosmic perspective you should actually turn your question around. The earth (fortunately) does not need energy to keep moving, the earth needs energy when you want to force it to stand still!

But where did the earth get the energy for its orbit around the sun today? That kinetic energy comes from the gas and dust cloud from which the solar system originated. I explain how that went in this video for Science101.