An artist's rendering of some of the forces of the universe. The apple falling is of course from the story of Isaac Newton discovering the law of gravity as an apple fell from a tree he was sitting underneath.
Windows to the Universe original image
Related links:Gravity is one of the universal forces of nature. It is an attractive force between all matter, and is very weak as compared to the other forces of nature. The gravitational force between two objects is dependent on their masses, which is why we can only see gravity in action when at least one of the objects is very large (like the Earth).
Isaac Newton was the first scientist to define gravity mathematically when he formulated his law of universal gravitation. The law of gravitation says that gravity is strongest between two very massive objects, and gets much weaker as these objects get further apart.
One of the applications of this law is the concept of escape velocity, which is the velocity an object needs to achieve to escape the gravitational pull of another object (like the Earth). Escape velocity can be calculated from Newtons gravitational law, and if we plug in the measurements we have for the planet Earth, we see that Earths escape velocity is about 11 km/s. This means that if you could throw a baseball at 11 km/s, it would never come down!
The concept of escape velocity is especially interesting when you consider black holes. These objects are extremely dense and very small. When we calculate the escape velocity for these objects, we find that the number is actually the speed of light, so not even light can get out of a black hole!