Chapter 14

Buoyancy

Buoyancy in a Liquid

•The downward weight of an object (force due to gravity) is not as great as an upward force on that object called the buoyant force buoyant force or buoyancyor buoyancy

Weight

Buoyancy in a Liquid

•The force of pressure is greater at the bottom of the object than the force of pressure exerted on the top of the object.

•Therefore, the buoyant force is due to the pressure differences between the top and the bottom of object.

Buoyancy arises from the fact that:

• fluid pressure increases with depth

• the increased pressure is exerted in all directions

• there is an unbalanced upward force on the bottom of a submerged object

Buoyant Force Equals Weight of Liquid

Displaced

• any object placed in water displaces a certain amount of water

• think about how the water level in the bathtub rises when you get in

• you can use the weight of the displaced water to determine the buoyant force

Buoyant Force Equals Weight of Liquid

Displaced

• What is the buoyant force on the weight?

• To calculate, figure out the weight of the displaced water.

Archimedes’ PrincipleArchimedes lived

over 2000 years ago in Greece. He discovered the fact that:

The buoyant force on an object is equal to the weight of the fluid displaced by the object.

Sink or Float?The buoyant force determines whether an object will sink or float.

If the buoyant force is greater than the weight of the object, then the object will float

If the buoyant force is less than the weight of the object, then the object will sink.

Sink or Float?Sink or Float?•An object floats when it displaces a volume of fluid whose weight is greater than or equal to its own weight.

•An object will float in a fluid if the density of that object is less than the density of the fluid.

Why is the tip of the iceberg the only part seen

out of the ocean?

IsostasyIsostasy•The buoyant force of the ocean pushes the iceberg upwards, but the volume of the ice is only slightly less than the same volume of salt water it displaces, so almost 90% of the iceberg remains submerged.

Why do some objects Why do some objects sink while others sink while others

float?float?•Density (the object’s mass

divided by it’s volume - how much space it takes up)

• In order for an object to float, the water it displaces must weigh more than the object itself

•Or to put in density terms, the object must have a density lower than the density of the water

If the density of If the density of water is 1g/cmwater is 1g/cm33, ,

then . . .then . . .•will wood (D = 0.8 g/cm3) float?•will aluminum (D = 2.7 g/cm3) float?

•will steel (D = 7.8 g/cm3) float?

Didn’t you just say steel would sink?!?

Then why is this ship made of steel floating on top of the water?

The shell of the ship may be made of steel, but most of the space inside the hull is filled with air that has a very low density.

Some creatures in the sea have gas filled bladders whose volume can be changed to adjust for the buoyant force at various depths.

This is how submarines work too. They can take in sea water to submerge or discharge sea water to rise up to the surface.

Can you think of something else that uses changes in the density within a chamber to make it rise or fall in our atmosphere?