Previously we have talked about the effects of gravity on objects near the earth's surface. However, Newton realized that it is more fundamental to devise a universal law of gravitation. He showed that the same force that keeps the planets in their orbits, is also responsible for keeping things on the Earth. In these notes we go into more detail about gravity, and try to explain why gravity works the way it does. You've lived with gravity all your life, and probably don't think much of it.
Gravity is what makes things fall, and what makes things heavy.
Newton claimed that any two objects with mass will exert a force on each other. Here on the surface of the Earth, we all feel the pull of gravity pulling us towards the ground. The Earth is so massive and so close compared to anything else, we can't feel the gravity from passing cars, airplanes flying overhead, or the planet Jupiter. That's why it seems to us that everything on the Earth tends to fall to the ground. The Earth's gravity still pulls on the spaceship and the astronauts almost as much as it does on people standing on the surface of the Earth.