Hi. It's Mr. Andersen and this is AP Physics essentials video 16. It is on density which
is the measure of the compactness of matter. And so what we have here are a number of fluids
that have different densities. At the bottom we have maple syrup, followed by dish soap
and water and wine and vegetable oil and olive oil. And so what do we know? Well the most
dense material, the maple syrup is going to be at the bottom. And the least dense is going
to be the olive oil at the top. And so matter has a property called density which measures
the compactness of that matter. And so it is affected by mass. And so let me show you
a couple of different objects. So we have object 1, object 2 and then object 3. And
so what those circles represent inside the square is they represent the atoms inside
that matter. And so if I were to ask you a question, which of these three has the greatest
mass your tendency is to say this one over here has the greatest mass. But if you look
at it they all have the same amount of atoms and so they are all going to have the same
mass. And so just mass alone does not tell us anything about the compactness. What do
we need? We also need to divide that mass by the volume. And so we set it up as an equation
where density is equal to mass over volume. And since the mass in all of these is remaining
constant, the larger the volume is, which is going to be in the denominator, than the
smaller our density is going to be. And so this would be the one with the smallest density.
And then this would be the one with the highest density. Or it is the most compacted of all
of the matter. And you should be able to calculate density given objects. And so if we look at
a bowling ball and a volleyball and I were to ask you which one has more density, well
you would know that that bowling ball has more material on the inside. If we were to
cut it open you would find that the volleyball is empty. It is filled with mostly air. But
that bowling ball is going to be filled with lots of matter on the inside. And so now even
though the volumes are the same, since the mass of the bowling ball is much higher it
is going to have a greater density. So you should be able to make predictions just by
looking at objects. And even looking at how objects change over time. What would happen
to the density of a volleyball as we heat it up and those molecules move apart? It would
become less dense. And as we cool it down it is going to become more dense. And so this
is a PHET simulation. What we are going to do is look at some different objects and figure
out what their density is. And so what we have here is a little bit of styrofoam. And
I am going to stop it right here. We can see that it has a mass of 0.75 kilograms. And
then a volume of 5.0 liters. And so how do you figure out the density? It is simply the
mass divided by the volume. And you can see the density right here is 0.15 kilograms per
liter. And so now let's change that from styrofoam to wood. You can see that the volume is staying
the same but the mass, since it is 2.0 and the volume is 5.0 we now have a density of
0.4. You will also see that it is still floating in this water that we have to the side. Now
let's switch that to ice. So if we look at ice what is our mass? The mass is 4.6 kilograms.
What is our volume? It is 5.0 liters. And so you can see that the density is 0.92. Now
it is approaching 1.0. And what has a density of 1.0? That is going to be the water itself.
And so since it is less dense than the water you can see the ice is floating. It is not
floating as high in the water as the wood and the styrofoam did. But it still is floating.
Now let's switch that to a brick and what happens there? You can see that our mass is
twice that of our volume. So now we have a density of 2.0 kilograms per liter. And it
definitely sinks. And then we could move it to something like aluminum and we have a much
higher mass compared to that volume and so we have a high density. So that is why it
sinks. Now what you should be able to do in this physics class is calculate the density
of an object. So I am going to object B and we are going to weigh it. We could figure
out what its gravitational mass is. It is 0.64 kilograms. And now I am going to use
water displacement. So you can see the level in the water is 100.00. But as I add object
B the level in the water goes up to 101.00. Now let's take object A, get its gravitational
mass. You can see it is 65.14. And now let's use that displacement of the water again to
figure out what is the volume of that cube. We can see that it went up to 103.38. And
so could you calculate the density of those two objects? And could you figure out if they
are going to sink or float? And so here is the important information that you need. You
can see that both of them started with a water level at 100.00 liters. And with object B
that rose to 101.00 liters and it had a mass of 0.64. And then with object A it rose to
103.38 and it had a mass of 65.14. And so you should pause the video right now and figure
out the density of object B and object A. If you forgot the equation it is going to
be right here. And you should also write next to that, is it going to float or is it going
to sink. So make sure you try this on your own. You should be able to almost do it in
your head. Alright. So hopefully you have figured out the density. And so now we are
going to throw those two objects in. And so we will throw in object B. And it floats.
So you should have gotten a density less than 1. And object A sinks. So it should have been
greater than 1. These are the values I got for density of object B and object A. And
you should be able to, at this point, predict the density of objects. You also should be
able to experimentally determine the density of objects. You could do it by measuring their
volume, figuring out the volume of a cube for example. Or you can use water displacement
to do it. And I hope that was helpful.