Buoyancy

If you drop an ice cube in air, it falls to the floor. If you drop the ice cube into water, ti may sink a little at first, but the cube quickly rises upward until it floats. You know that gravity is pulling downward on the ice, even when it is in the water. If the ice cube is not sinking, there must be some force balancing gravity that is pushing upward on it.

The upward force on objects in a fluid is called  buoyant force, or buoyancy. Buoyancy is why ice floats in water. Because of buoyant force, objects seem lighter in water. For example, it is easier to lift a heavy rock in water than on land because the buoyant force pushes upward on the rock, reducing the net force you need to lift it.

Buoyancy

The photograph below shows a balloon that has been pushed into a beaker of water. Remember that in a fluid, pressure increases with depth. This means that there is greater pressure acting on the bottom of the balloon than on the top of it.  The pressure difference between the top and bottom of the balloon produces a net force that is pushing the balloon upward.

When you push a balloon underwater, the water level rises because the water and the balloon cannot be in the same place at the same time. The volume of the water has not changed, but some of the water has been displaced, or moved, by the balloon. The volume of the displaced water is equal to the volume of the balloon. The buoyant force on the balloon is equal to the weight of the displaced water. A deflated balloon would displace less water and would therefore have a smaller buoyant force on it.

Density and Buoyancy

Whether or not an object floats in a fluid depends on the densities of both the object and the fluid. Density is a measure of the amount of matter packed into a unit volume. The density of an object is equal to its mass divided by its volume and is commonly measured in grams per cubic centimeter.

If an object is less dense than the fluid it is in, the fluid the object displaces can weigh more than the object. A wooden ball that i pushed underwater,a s in the beaker below, rises to the top and floats. An object rising in a liquid has a buoyant force acting upon it that is greater than its own weight. If an object is floating in a liquid, the buoyant force is balancing the weight.

If the object is more dense than the fluid it is in, the object weighs more than the fluid it displaces. A glass marble placed in the beaker on the far right sinks to the bottom because glass is denser than water. The weight of the water the marble displaces is less than the weight of the marble. A sinking object has a weight that is greater than the buoyant force on it.

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