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. Click on image for full size (42K GIF) Windows Original

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 Newton’s gravitational law, and if we plug in the measurements we have for the planet Earth, we see that Earth’s 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!