If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:4:03

in this video we're going to talk a little bit about density and we're especially going to talk about density in the context of volume and one simple way to think about density is it's a quantity of something and we're going to think about examples of it per unit volume so per volume so for example let's say that i have a cubic meter right over here actually let me have two different cubic meters just to give you an example so these are both cubic meters and let's say in the one on the left i have a quantity of let's say six of these dots per cubic meter and over here i only have three of these dots per cubic meter well here i have a higher density and in general we're going to think take the quantity and divide it by the volume and the units are going to be some quantity per unit volume now you're typically going to see mass per unit volume but density especially in the volume context can refer to any quantity per unit volume now with that out of the way let's give ourselves a little bit of an example so here we're told that stone spheres thought to be carved by the decay people i'm not sure if i'm pronouncing that correctly more than a thousand years ago our national symbol of costa rica one such sphere has a diameter of about 1.8 meters and masses about 8 300 kilograms based on these measurements what is the density of this sphere in kilograms per cubic meter round to the nearest hundred kilograms per cubic meter so pause this video and see if you can figure that out all right so we're going to want to get kilograms per cubic meter so we know the total number of kilograms in one point in a sphere that has a diameter of 1.8 meters so that's the total number of kilograms but we don't know the volume just yet so we have a sphere like this this would be a cross section of it its diameter is 1.8 meters now you may or may not already know that the volume of a sphere is given by four thirds pi r cubed and so the radius here is 0.9 meters and so that would be the r right over here so the volume of one of these spheres is going to be let me write it over here the volume is going to be four thirds pi times 0.9 to the third power and we know what the mass is the mass in that volume is 8 300 kilograms so we would know that the density the density in this situation is going to be 8 300 kilograms 8 300 kilograms per this many cubic meters four thirds pi times 0.9 to the third power cubic meters and we're going to need a calculator for this and we're going to round to the nearest hundred kilograms so we have 83 hundred divided by and let me just open parentheses here four divided by three times pi times .9 to the third power and then i'm going to close my parentheses is equal to this right over here we want to round to the nearest hundred kilograms so approximately 2700 kilograms per cubic meter 2700 or 2700 kilograms per cubic meter and we are done