Gravity is one of those things we take completely for granted. And there are twothings about it that we take for granted: the fact that it is always there, and the fact thatit never changes. If the Earth's gravity were ever to change significantly, it would have ahuge effect on nearly everything because so many things are designed around the currentstate of gravity.
    Gravity is an attractive force between any two atoms. Let's say you take two golfballs and place them on a table. There will be an incredibly slight gravitational attractionbetween the atoms in those two golf balls. If you use two massive pieces of lead andsome amazingly precise instruments, you can actually measure an infinitesimal attractionbetween them. It is only when you get a gigantic number of atoms together, that the forceof gravitational attraction is significant.
    The reason why gravity on Earth never changes is because the mass of the Earthnever changes. A change in mass great enough to result in a change in gravity isn't goingto happen anytime soon.
    But let's ignore the physics and imagine that, suddenly there was no force of gravityon planet Earth. This would turn out to be a pretty bad day. We depend on gravity to .holdso many things down--cars, people, furniture, pencils and papers on your desk, and soon. Everything would start to float. What's more, two of the more important things heldon the ground by gravity are the atmosphere and the water in the oceans, lakes and rivers.
    Without gravity, the air in the atmosphere would immediately leap into space. This is theproblem the moon has--the moon doesn't have enough gravity to keep an atmospherearound it, so it's in a near vacuum. Without an atmosphere, any living thing would dieimmediately and anything liquid would boil away into space.
    In other words, no one would last long if the planet didn't have gravity.
If gravity were to suddenly double, it would be almost as bad, because everythingwould be twice as heavy. There would be big problems with anything structural. Houses,bridges, skyscrapers, table legs and so on are all sized for normal gravity. Most structureswould collapse fairly quickly if you doubled the load on them.
    What this answer shows you is just how integral gravity is to our world. We can'tlive without it. It is one of the true constants in our lives.