Fluids Part 3A look at buoyancy and ballast

Mr. Williams

Grade 8 Science

BuoyancyHave you ever tried to put a wooden boat on a pond or

seen a log float down a river. Why do these objects float? The answer is due to the property of buoyancy.

Buoyancy – Is the upward supportive force on an object in a fluid.

A block of solid metal will sink in water, yet a boat that is made of the same amount of metal will not?

Why does this work for the boat, but not for the metal block?

Why does the shape of the metal determine whether it will sink or float?

Buoyancy Cont’d

The force of gravity acting downward on an object is equal to the buoyant force of the water acting up on the object

The design of many ship foils determines how much cargo they can carry. This is why large tankers are able to carry so much weight.

Buoyancy Cont’d

We have learned that fluids have an upward push known as buoyant force, however, different fluids exert different forces.

To measure this we use hydrometers to see the level of buoyant force each fluid exerts.

Many cargo ships have a plimsoll line painted on the ship to show how the ship will float in different parts of water.

Plimsoll Line

Density and Buoyancy in Action

Density and buoyancy are very closely related characteristics of fluids that help us understand how we can use fluids in our lives

Oil and Water:

Oil is usually less dense than water and so it floats on the surface of water. This is helpful because in the event of a spill it helps us be able to clean it off the surface.

Yet, oil can destroy shore habitats and many plants and animals

Airships, Balloons and Blimps:

Old airships use to be filled with hydrogen gas. This was less dense and less expensive than helium gas, but far more explosive.

Modern day air balloons and blimps use either heated air or helium to move the balloon.

The heated air inside the balloon causes the balloon to rise because it is less dense than the cooler air outside.

Airships, Balloons and BlimpsThe Hindenburg Disaster in 1937

Modern Day Balloons

The Importance of Buoyancy

Most bony fish maintain their position in the water with the use of a swim bladder. This is a thin walled sac in their bodies that contains mainly oxygen.

Swim bladder – A controllable balloon-like chamber that allows fish to alter their buoyancy.

Fish can alter the volume of water they displace by adjusting the amount of oxygen flow in the bladder. The more oxygen in the bladder, the higher they float.

Swim Bladder

Submarines and Ballast

Submarines work in a very similar fashion to fish and their swim bladders.

Submarines contain ballast tanks in place of bladders.

Ballast tanks - are specialized chambers that can be filled with air or water to control the depth of the submarine.

To make the submarine descend, valves are opened to take on more water in the ballast tanks. To rise, air from compressed air tanks is forced into the ballast tanks which displaces the water out of the submarine.

Submarines and Ballast

Buoyancy Cont’d

By controlling the amount of air in these chambers, both fish and submarines can change their density causing positive, negative or neutral buoyancy.

When positive buoyancy occurs, objects begin to float upwards.

When negative buoyancy occurs, objects begin to sink

When an object remains in the same place it is neither sinking or rising, therefore it has neutral buoyancy.