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, which says that the force of gravity between two objects (F) equals the mass of one object multiplied by the mass of the second object multiplied by the Gravitational constant (G, equal to 6.67 x 10^-11 Newton meters ^2/kg^2), all divided by the square of the distance between the two objects. This means that gravity is strongest between two very large 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 is equal to the square root of the quantity 2GM/R, where M is the mass of the object you’re trying to escape from, G is the Gravitational constant, and R is the radius of the object you’re escaping from. 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, so M is huge and R is tiny. 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! The size of these objects is almost unbelievable--if the Earth were to collapse so that it was this dense, it would end up being only about 2 cm in diameter!

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!

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 using math. There will be a larger pull due to gravity if the objects' masses are larger and if the objects are closer together.

One thing we can do with Newton’s law is calculate an escape velocity for the Earth. That is the speed something has to go for it to escape the gravity of Earth. This number for Earth is about 11 km/s. This means that if you could throw a baseball at 11 km/s, it would never come down!