Density and Buoyancy

Changes in Density We know as temperature increases, density decreases. Why?

Changes in Density We know as temperature decreases, density decreases. Why?

Example: Hot Air Balloon

Buoyancy Buoyant Force: is the upward force on objects submerged in or floating on liquids

Floating Gravity + Buoyant Force = no movement (rest) Example: Boat

Sinking Gravity + Buoyant Force = Downward Motion Example: Rock

Rising Gravity + Buoyant Force = Upward Motion Example: Hot Air Balloon

Archimede’s Principle The Buoyant Force is equal to the weight of the displaced water

Salt Water vs. Fresh Water Salt 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 fluid Salt water can support more weight per volume and it is easier to float

The Dead Sea – 31% salinity The Dead Sea in Jordan is 9 times saltier than all oceans on the planet

Average Density When 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: Pencil D = m wood +m rubber +m lead +m aluminum volume This is why objects that you think would normally sink are able to float

Ships Hollow hulls

Life Jackets Made with low density materials Decreases a person’s average density therefore they float

Submarines Use the flow of water to change the density of the submarine When the weight = buoyant force floats When 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 page Write a response to the 3 questions at the end of the video in your notebook