According to Newton's law of gravity, each object exerts an attraction on the things around it. Since the Earth is millions of times bigger than the things on it, everything pulls it towards itself. If it were not so, everything on Earth, including people, would be shot into space.
Why, then, when a ship is on the water, does it not sink? After all, its structure, machinery, cargo, crew and passengers add up to a weight of thousands and thousands of tons.
The reason is that all ships have been designed with hollow areas that are filled with air. And while a ship pushes some of the water to the sides when it floats, the large amounts of air inside weigh less than the water that is displaced. The water beneath it, however, weighs much more and pushes the ship up, which makes it float.
Examples of buoyancy can be seen in beach balls, if one pushes the ball into the water, it reacts by pushing the ball up.
What determines whether an object floats or sinks is its density, the amount of air inside it. A large piece of wood floats, because the air fills many of its spaces, but a small stone will sink because it has a greater density, with no air inside.
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