How does gravity make things float?
A balloon of helium gas will float high, high in the air. A cork and even a lump of foamy pumice stone will bob on the surface of the water. An iron poker will sink through water, but it will float on a pool of mercury. The force of gravity plays a part in these floating tricks, but so do other forces.
A buoy is a floating marker to warn ships at sea and buoyancy is the floating quality which makes balloons rise in the air and lifebelts bob on the waves. Buoyancy gets its bouncy energy from a combination of two natural forces, gravity and pressure. Mercury is denser than iron which is why gravity exerts a greater force upon it, making it a heavier material. A pool of mercury also exerts pressure under a bar of iron. These two forces working together make the iron float on the surface of the liquid mercury.
Gravity is a mysterious force of attraction built into every object. The more massive an object, the more gravity it exerts. The mass of an object is the amount of matter packed into a certain space. A pail of coal is more massive than a pail of feathers because it is denser. It has more atoms and molecules packed closer together than the fluffy, feathers.
Every object exerts some gravity on every other object. But compared with the gravity of the massive earth, even the gravity of a big mountain counts for almost nothing. The weight of a pound of cheese, or anything else, is the measure of the attraction of gravity between the cheese and the earth. Your weight is a gift from the earth, but your ability to float in the salty sea borrows some of the pressure exerted by the surrounding water.
An astronaut might weigh 150 earth pounds, but when he lands on Mars, he will weigh only 54 pounds. If he lands on Jupiter, perish the thought, he will weigh 406 pounds and sink down through the soupy ices which, we are told, may cover the surface of the giant planet. But a feather might float there because the upward pushing pressure from the ices would be stronger than its weight.
Maybe you have tried to lift a big stone from the bottom of a stream. You get it to the surface, and then find it too heavy to carry. No, it did not gain any weight. While you were lifting it through the stream, the water exerted pressure on its entire surface. The pressures around the sides canceled each other out, leaving only a pushing pressure from below. This pressure helped you lift the stone through the water and made it seem lighter. The same combination of gravity and pressure provide the buoyancy which makes a balloon rise, a cork bob on the water and a bar of iron float on a pool of mercury.
Designers of ships and aircraft must calculate the quality of buoyancy to the finest fraction. They must know the density of the object they want to float and the exact pressure of the substance around it. In designing space craft, there is another factor. The force of gravity diminishes with distance and out in the void of space a ship is weightless because it has escaped the pull of the eartbl3 gravity.