Lesson video

Hi, my name is Mrs. Dennett.

And in this lesson we're going to be using and applying the density formula.

Let's start by thinking about, what is density? Density is the amount of mass per unit of volume.

That means that it is another example of compound measures, because we've got the mass and the volume.

So we've got two different units of measurement involved here.

So an example of density is kilogrammes per centimetre cubed.

So here we've got the kilogrammes, which is the mass, and we've got the centimetre cubes or cubic centimetres, which are a unit of volume.

And we're measuring the kilogrammes per cubic centimetre.

We calculate density using the following formula, density is equal to mass divided by volume.

And I like to use the measure of density, the compound measure kilogrammes per centimetre cubed, which is a common measurement of density.

And this helps with me to remember the formula, because you can see that you've got kilogrammes, which is the mass, that lie per which means divide.

So we've got mass divided by centimetres cubed, which we should know is a volume.

So that's how I like to remember the formula.

I like to use the units involved density equals mass divided by volume.

And the units for density are always a unit of mass per unit of volume.

So let start to use the density formula.

I've written it there for your, density equals mass divided by volume.

A box has a massive 500 grammes and a volume of 125 centimetres cubed, workouts it's density.

So the first thing we do is write down the formula and then we have a look at the information we're being given in the question.

So we've got 500 grammes, which is a measure of mass, and we've got a volume measure of 125 centimetres cubed.

So we put those values into our formula.

And we do 500 divided by 125.

So that tells us that the density is four.

And then let's think about these units here.

We've divided grammes by centimetres cubed.

So the answer will be four grammes per centimetre cubed.

Let's have a look at a question where we want to calculate the mass.

We can still use density formula because we're given that the density of the cube is 25 grammes per millimetre cubed.

And you can see a picture of our cube there.

And on the diagram we can see that a side length of the cube is 10 millimetres.

So let's write down our formula and put the information that we've got from this question into our formula.

So we know, that the density is 25 grammes per millimetre cubed.

And we can see that the volume can be worked out by multiplying the base by the height, by the length.

So we've got 10 times, 10 times 10.

So the volume is a thousand millimetres cubed.

So let's put these values into our formula.

Now that we know the volume, so we do the density is 25 and that's equal to the mass which we're trying to find, divided by a thousand.

So we rearranged this formula and to find the mass, we do 25 times by a thousand, which gives us a massive 25,000 grammes because we know, that the density was measured in 25 grammes per millimetre.

There some questions for you to try.

Pause the video to complete the tasks and restart when you were finished.

Here are the answers.

Remember that density is mass divided by volume.

So we're looking for units that have a unit of mass per unit of volume.

This is kilogrammes per metres cubed and grammes per centimetre cubed.

The first one, in fact, the first two, can't be right as they're not compound measures, and the last one, grammes per centimetre squared, well sends me to squared is an area, not a volume, so that can't be right.

To complete the table, you just needed to use the density formula, rearranging it for the bottom four rows as you're calculating aither mass or volume.

Here are some questions for you to try.

Pause the video to complete the tasks and restart when you were finished.

Here are the answers, for question three, you just need to substitute the mass 45 grammes and the volume 5.

3 centimetres cubed into the formula.

45 divided by 5.

3 is 8.

5 grammes per centimetre cubed to one decimal place.

For question four, you needed to work out the volume of the cuboid first, this is four times three times 14, which is 168 centimetres cubed.

Then calculate density, and we get 2.

7 grammes per centimetre cubed.

That's all for this lesson.

Remember to take the exit quiz before you leave.

Thank you for watching.