Buoyancy is the force exerted on an object by a fluid and is usually acts against gravity. This is why objects feel lighter in water. Some of the force of gravity (weight) is cancelled out by the force of buoyancy. If the buoyant force is stronger than the force of gravity, the object will float in the fluid.

Gravity

Buoyant Force

Archimedes’ Principle states that the buoyant force of a fluid is equaled to the weight of the fluid displaced by the object. If an object can displace (push away) enough fluid to equal its weight, it will float. This is why a tremendous ship made of iron can float but a cannonball would sink. The ship’s shape allows it to push enough water away to equal the weight of the ship.

5.0Pounds

0.0 lbs5.0 lbs

If the object can push away an equal weight of fluid as its own weight, it will float in that fluid.

10.0Pounds

0.0 lbs6.0 lbs

If the object pushes away a lower weight of the fluid than its own weight, it will sink in that fluid.

The amount of water that was displaced by the floating boat weights the same as the boat itself. According to Archimedes’ Principle, this is why the boat floats and doesn’t sink.

Displaced Water

Sinkable Rock

If the rock is thrown overboard, how will the water level in the tub change?

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