Unit 2 Fluids

Buoyancy

and

Archimede’s Principle

Buoyancy and Buoyant Forces

When you are swimming inwater, there are two forces thatwork against each other andaffect the motion of your body.

the force of gravity is pulling you down

the water is also pushing you up with a buoyant force

Bouyancy

Buoyant force, or buoyancy, is the upward force on objects submerged in or floating on fluids.

A buoyant force pushes away from the centre of Earth.

Bouyancy

Bouyancy

 an object will float if its buoyant force, when fully immersed, is greater than its weight (gravitational force)

it will sink if its weight is greater than the buoyant force

it will float when the buoyant force is equal to its weight (or the force of gravity)

Archimede’s Principle

This principle explains why some objects float in water and others sink.

Salt vs. Fresh water

Seawater (salt water) has a density of 1.03 g/mL and fresh water has a density of 1.00 g/mL.

Therefore, one litre of salt water weighs more than one litre of fresh water.

That is, salt water can support more weight per volume than fresh water, so it is easier to float in salt water.

Average Density

The average density of an object is the total mass ofall substances that make up the object divided by the total volume. Average density results in objects that would

normally sink being able to float.

Examples of technologies that have been developed because of our understanding of density and buoyancy include:

1. Ships

Ships can be built of steel

because their hollow hull

ensures that the average

density of the ship is less

than that of water.

2. Personal Floatation devices (Life jackets)

Personal flotation devices (ex.Life jackets) are filled with a substance of very low density.

This way, a life jacket lowers a person’s average density, allowing the person to float.

3. Submarines

o By allowing water to flow in or out, a submarine can rise or sink in the water.

o The submarine floats when its weight is equal to the buoyant force and it sinks when its weight is greater than the buoyant force.

4. Hot Air Balloons

When the air inside a hot-airballoon is heated, the air particles: • gain energy and • spread out (forcing some of the

particles out of the balloon)

The air inside the balloon becomesless dense than the air surroundingit, so it rises.

Average Density

So: an object will float if its average density is less

than the fluid in which it is immersed

an object will sink if its average density is denser than the fluid in which it is immersed

when the object’s density is the same as the medium, an object will neither sink nor float; it is said to be neutrally buoyant.

Sink or Float?

wooden boats vs. a water logged stick

metal block vs. metal boats

a sealed, empty plastic bottle vs. a plastic bottle full of water