DensityDensity

A ton of feathers A ton of feathers or a ton of bricks?or a ton of bricks?

Which would take up more space? Which would take up more space?

What about the size of that space? What about the size of that space?

What about the amount of “stuff” What about the amount of “stuff” or or mattermatter in that volume? in that volume?

Density is: Density is: The ratio between the mass of an object The ratio between the mass of an object and its volume (another way of saying and its volume (another way of saying how much “stuff” there is in a given how much “stuff” there is in a given volume)volume)

Or, as a formula: Density = mass/volumeOr, as a formula: Density = mass/volume

Sometimes expressed using “rho,” the Sometimes expressed using “rho,” the Greek letter that looks like a p, so you Greek letter that looks like a p, so you may see may see p=m/vp=m/v

What else about What else about Density?Density?

Depends upon how many particles are in Depends upon how many particles are in a given space. a given space.

For example, the density of mercury is For example, the density of mercury is 13.5 times that of water – for the same 13.5 times that of water – for the same volume, mercury has 13.5 times more volume, mercury has 13.5 times more “stuff” in it.“stuff” in it.

It’s a derived unit – that means that its It’s a derived unit – that means that its measurement comes from two or more measurement comes from two or more other measurements, and as such, it has other measurements, and as such, it has a unique unit. a unique unit.

Units of DensityUnits of Density““Derived Unit” – so what are the units Derived Unit” – so what are the units of density? of density?

Mass units – gramsMass units – grams

Volume – Volume – eithereither mL mL oror cm cm33 depending on depending on liquid or solid. liquid or solid.

Units are grams/mL (liquids) OR Units are grams/mL (liquids) OR grams/cmgrams/cm33 solid solid

Occasionally unitless. Occasionally unitless.

Can Density Can Density change? change?

Yes.Yes.

What happens to the particles when you What happens to the particles when you heat something? heat something? Think back to solid-Think back to solid->liquid->gas…>liquid->gas…

What changes in density=mass/volume? What changes in density=mass/volume?

Or rather, Or rather, howhow can we change the mass can we change the mass or volume of an object? or volume of an object?

Changing Density:Changing Density:A lava lamp (waxy goo heated by light A lava lamp (waxy goo heated by light at bottom – volume changes)at bottom – volume changes)

A submarine (water pumped in and out A submarine (water pumped in and out – changing the sub’s mass)– changing the sub’s mass)

Hot air balloon (heat the air – the Hot air balloon (heat the air – the volume of air inside the balloon stays volume of air inside the balloon stays the same, the mass changes)the same, the mass changes)

Density is a Density is a Physical Property Physical Property

of Matterof MatterIT CAN CHANGE – know what can change IT CAN CHANGE – know what can change it. it.

How can we How can we measure density? measure density?

Simple – mass over volume. Simple – mass over volume.

How do we measure mass? How do we measure mass?

How do we measure volume? (2 ways)How do we measure volume? (2 ways)