# Can we go slower than the earth

## Moon, earth and gravity

06 June 2016

Have you ever seen a fruit or object fall from a tree? Have you ever thrown a stone and watched it fall? The force that makes objects “fall” is called gravity.

It is also gravity that pulls us steadily towards the earth. Because of gravity, our feet stay on the ground. We don't have to touch the earth directly to be attracted to it. Gravity works as long as we are not too far away. Due to gravity, the earth orbits the sun and the moon orbits the earth.

Gravity depends on the mass of an object. The force of gravity acting between two objects is proportional to the mass of the objects and decreases very quickly as these objects move away from each other. We ourselves also attract objects with “our” gravity, but we have far too little mass to be able to recognize an effect! The sun, on the other hand, is so huge that it holds the earth tight with its gravity even though it is so far away. The moon also has gravity. Since it is smaller and lighter than the earth, we would weigh only one sixth as much on the moon as we do here on earth.

One can, of course, wonder why the moon doesn't fall to earth like an apple from a tree. That's because the moon never stands still. He is constantly moving around the earth. Without the earth's gravity, it would float off into space. Because of this mixture of speed and distance from Earth, the moon is in a permanent balance between falling and escaping. If it were faster, it would float away. If he were slower, he would fall to the ground.

As we said before, gravity also depends on distance. If we could move far enough from the earth, we would escape its gravity. That's exactly what we're trying to do with spaceships. When taking off, they must reach at least the so-called escape speed of 11.2 km / s. (At this speed it would only take 10 minutes to travel from London to New York!) When a spaceship reaches the escape speed, it overcomes the gravitational force of the earth and gets into the solar system.

In a spaceship or a space station in orbit, we do not feel the earth's gravity. Objects don't fall down, they float. When you jump up, you don't fall back on the ground. This is exactly what happens to astronauts in a space station orbiting the earth!