The Force of Gravity Any two objects that have mass attract each other with a force we call gravity. You probably never noticed this for small objects, because the force is so weak. But the Earth has lots of mass, and so it exerts a big gravitational force on you. We call that force your weight. The fact that gravity is actually a force of attraction is not obvious. Prior to the work of Isaac NEWTON, it was assumed that gravity was simply the natural tendency of objects to move downward. If you weigh 150 lb, and are sitting about 1 meter (3.3 feet) from another person of similar weight, then the gravitational force of attraction between the two of you is 10-7 lb. This seems small, but such forces can be measured; it is about the same as the weight of a flea. You weigh less when you stand on the Moon, because the force of attraction is less. If you weigh 150 lb on the Earth, you would weigh only 25 lb on the Moon. You haven’t changed (you are made up of the same atoms), but the force exerted on you is different. Physicists like to say that your mass hasn’t changed, only your weight. Think of mass as the AMOUNT of material, and weight as the force of attraction of the Earth (or whatever other planet or SATELLITE you are standing on). Mass is commonly measured in kilograms. If you put a kilogram of material on the surface of the Earth, the pull of gravity will be a force of 2.2 lbs. So a good definition of a kilogram is an amount of materialBut if you are so small, how can you exert such a large force on the Earth? The answer is that, even though you are small, your mass exerts a force on EVERY piece of the Earth, simultaneously. When you add all those forces together, the sum is 150 lb. So you are pulling up on the Earth exactly as much as the Earth is pulling down on you. Think of it in the following way. If you push on some else’s hand, they feel your force. But you feel the force too. You push on them; they push back on you. The same thing WORKS with gravity. The Earth pulls on you; you pull on the Earth.