Gravity is still
considered a great mystery in the universe. The theory which best explains gravity is that every single mass in the universe pulls on every other mass, and the strength of that pull is affected by the distance. The higher the mass, the greater the pull. The greater the he distance, the less the pull. But why does every object in the universe pull on every other object? That still remains a mystery.
In everyday speech, mass and weight are used interchangeably, but they are actually quite different. Mass measures the amount of material in an object. A pillow and a paperweight might have the same mass, although they differ in size. The paperweight is more dense than the pillow; the material inside the paperweight is packed more tightly.
Weight measures to the force exerted on an object in a gravitational field. Mass doesn't change due to location, but weight can change depending on the environment. A mass on the surface of the earth experience a pull from the planet of 9.8 meters per square second. If the same mass were taken to the surface of the moon, the object would actually weigh less because the moon is less massive than earth. The moon has one-sixth the gravitational pull, so the object would weigh one-sixth as much. In other words, a 150 lbs man would weigh only 25 lbs on the moon.
Our bodies are designed to exist under the force of gravity from the earth. When astronauts experience weightlessness in space- more correctly called microgravity- they experience nausea, disorientation, headaches, loss of appetite, and congestion. They become so spacesick. Normally, the blood that flows into your legs had to resist gravity to flow back up to your heart. In microgravity, the blood rushes back up to your brain because there is no resistance. It's like hanging upside down for a long period.
Astronauts have to exercise for some hours a day in space to prevent muscles from atrophying.
Sir Isaac Newton wrote the first mathematical formation of gravity in 1687.
The legend that Newton conceived the idea of gravity after an apple fell from a tree has no merit.
Sources and further readings
Stanford University
NASA