Density and Buoyancy

Changes in DensityWe know as temperature increases, density

decreases.Why?

Changes in DensityWe know as temperature decreases, density

decreases.Why?

Example: Hot Air Balloon

BuoyancyBuoyant Force: is the upward force on

objects submerged in or floating on liquids

FloatingGravity + Buoyant Force = no movement (rest)

Example: Boat

SinkingGravity + Buoyant Force = Downward Motion

Example: Rock

RisingGravity + Buoyant Force = Upward Motion

Example: Hot Air Balloon

Archimede’s PrincipleThe Buoyant Force is equal to the weight of

the displaced water

Salt Water vs. Fresh WaterSalt Water D= 1.03 g/mL Fresh Water D= 1.00 g/mL

Therefore, mass of 1L salt water > mass of 1L fresh water

This means:The larger the mass, the larger the buoyant

force acting on an object submersed in the fluidSalt water can support more weight per volume

and it is easier to float

The Dead Sea – 31% salinityThe Dead Sea in Jordan is 9 times saltier than

all oceans on the planet

Average DensityWhen items are made up of different

substances it can be hard to know exact densities, so instead average densities can be used

Average Density = total mass of all substances total volume

Example: PencilD = mwood+mrubber+mlead+maluminum

volumeThis is why objects that you think would

normally sink are able to float

ShipsHollow hulls

Life JacketsMade with low density materialsDecreases a person’s average density

therefore they float

SubmarinesUse the flow of water to change the density

of the submarineWhen the weight = buoyant force

floatsWhen the weight > buoyant force

sinks

Key Ideas

Object average density < fluid density FLOAT

Object average density > fluid density SINK

Object average density = fluid density NEUTRALLY BUOYANT

* “fluid” can refer to a liquid or gas

Assignment:Watch the “Density and Buoyancy” Video

posted on the class Science pageWrite a response to the 3 questions at the

end of the video in your notebook