Upload
baldwin-dorsey
View
227
Download
3
Tags:
Embed Size (px)
Citation preview
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
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)
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.