Why do boats float and rocks sink? When an object is placed in water, there are two primary forces acting on it. The force of gravity yields a downward force and a buoyancy force yields an upward force. The gravitational force is determined by the object's weight, and the buoyancy force is determined by the weight of the water displaced by the object when it is placed in water. If the gravitational force is less than the buoyancy force then the object floats (a boat), otherwise it sinks (a rock). That is, if an object weighs less than the amount of water it displaces then it floats otherwise it sinks. Read on for a more detailed explanation.
Fact 1.	Fact 2.	Fact 3.
When an object is placed in water, it will displace water to "make room" for the object (e.g., when you get into a bath, the level of the water rises).	When an object is submerged in water, the surrounding water exerts a force (buoyancy force) on the object. This force increases with the depth of the water, so that for any submerged object, there is a net force upwards.	Any object is attracted downward due to gravity. This force increases with the mass of the object.
There are two primary forces acting on an object placed in water: a gravitational force acts in the downward direction causing the object to sink. The strength of this force depends on the object's mass (weight) -- the more massive an object the stronger the downward gravitational force will be. a buoyancy force acts in all directions, but has a net upwards direction, causing the object to float. The strength of this force depends on how much water the object displaces -- the more water that is displaced the stronger the upward buoyancy force. If the downward gravitational force is less than the upward buoyancy force then the object floats, otherwise it sinks. That is, if an object weighs less than the amount of water it displaces then it floats otherwise it sinks. A boat floats because it displaces water that weighs more than its own weight.

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