Have you ever seen a fruit or object falling from a tree? And have you ever tried to throw a stone and watch it fall? This force that ‘brings down’ things is called the force of gravity.
We are constantly attracted to the Earth by its gravitational force. That is the reason why we always keep our feet on the ground. We don’t need to be in direct contact with Earth to be attracted by it however; not being too far away is just enough for the same forces to act. This is why our own planet orbits around the Sun, and the Moon orbits around the Earth.
The gravitational force is determined by the mass of an object. The gravitational force among two objects is proportional to the mass of the objects, and it decreases very fast the moment we separate them. In fact we also attract objects with ‘our’ force of gravity, but we’re too lightweight to see the effects! The Sun, however, is so big that it’s able to hold us close even when it’s so far away. The Moon also exerts its force of gravity; since it’s smaller and lighter than Earth, if we could weigh ourselves on it we would discover we weigh around a sixth of our weight on Earth.
One could ask why the Moon doesn’t fall on Earth as an apple from the tree. The reason is that the Moon is never still. It constantly moves around us. Without the force of gravity from the Earth, it would just float away into space. This mix of velocity and distance from the Earth allows the Moon to always be in balance between fall and escape. If it was faster, it would escape; any slower and it would fall!
We said the force of gravity depends on distance too. If we were to distance ourselves enough, we could escape its hold. That’s what we try to do with spacecrafts. We need to reach and exceed the so-called ‘escape velocity’, that is about 11.2 km/s (at such velocity, we would be able to move from London to New York in just ten minutes!). Once a shuttle reaches this velocity, it is free to travel in the Solar System.
Inside an orbiting shuttle we do not feel the gravitational force of the Earth. Objects don’t fall, they float; if you jump up, you don’t come back down. The same thing happens to astronauts when they are in space stations orbiting the Earth!