About this video
Explains why a ton of bricks and a ton of feathers do not feel like they weigh the same, even though the total weight is equal. The key idea is density, which is how much mass is packed into a given volume. Density lets us predict whether an object or liquid will float or sink.
You'll be able to
- define density as mass packed into a given volume
- predict whether an object will float or sink using density
- explain why comparing equal weights and comparing equal volumes give different answers
Key words
- Density
- How much mass is packed into a given volume.
- Mass
- The amount of matter an object contains.
- Volume
- The amount of space an object or substance takes up.
- Float
- Stay on top of a liquid because the object is less dense than the liquid.
- Sink
- Fall through a liquid because the object is denser than the liquid.
Transcript
- [Instructor] Here's an old joke. What weighs more? A ton of bricks or a ton of feathers? When I first heard this, I was like, of course, bricks weigh more than the feathers. Come on, that's obvious. But wait, we asked what weighs more? A ton of bricks or a ton of feathers?
Since they're both one ton, they must weigh exactly the same. But this feels weird. My brain is telling me bricks must weigh more than the feather. Why is that? Well, because when I hear this question, I completely ignore the one ton part, and I consider them having equal volumes.
I consider equal volumes of bricks and feather, and if you do consider equal volumes of bricks and feather, then of course bricks will weigh more than the feather. But the question asked one ton of bricks and feather. Let's see, what would that look like? If you took a ton of bricks, it'll occupy a space, which is almost the same as a household refrigerator.
But if you take a ton of feather, (chuckles) you would need as as much, it'll occupy a space of a small house, okay? Only then they will have the same mos. So clearly this is very weird, and we usually don't think like that. So we completely ignore the ton part, and we assume instinctively that they have the same volume, equal volume, and therefore we conclude that bricks will weigh more than the feathers.
Or we conclude that, hey, metals are heavier than plastic, for example, with instinctively thinking that they have the same or equal volume. And this is interesting because what we are instinctively thinking about is something called the density. What exactly is density you ask?
Well, density is a measure of how much mass is packed in a given volume. So bricks have a higher density compared to feathers, because in bricks, more mass is packed in a given volume compared to feathers. Here's another way to visualize this. We can clearly see, you know, particles are more crowded over here compared to over here, right?
So they're packed more closely over here. Now, if all particles were identical, if each particle had the same mass, then you have more mass packed in a given volume compared to over here. So higher density. Now it's possible that this material has more particles, this material has more mass compared to this material.
That's totally possible because this material can be very, very big. But density is not about mass. It's about how much that mass is packed. That's why. This has higher density, because that mass is packed very tightly. But why care about density? What can we do with this knowledge?
Well, a cool thing is we can predict whether something is going to float or sink. For example, the density of iron is higher than that of water. That's why if you take a nut or a bolt, which is mostly made of ion, if you put it in water, it will sink. But look at ice, it floats, why?
Because the density of ice is lower than that of water. That's why it floats. And this is not just for water. This will work for any liquid. If the density of that object is higher than the density of that liquid, it'll sink. If it's lower, it will float. Here's another example.
Density of ion is lower than that of mercury. Mercury is a liquid at room temperature, which means if you were to put this same ion bolt in, mercury, a glass of mercury, it would float on top of it, and that's exactly what you see over here. How cool is that? You have iron floating on top of a liquid, and what's cooler is that this is not only true for solids in liquids.
This also works when you're considering two liquids. For example, if you take oil and water, density of oil is lower than that of water. So if you pour oil and water, you would expect oil to float on top of water, or we can save water sinks in oil because it has a higher density compared to oil.
So long story short, density is a measure of how much mass is packed in a given volume. More the mass packed in a given volume, higher the density. And it's very useful to think about whether stuff is going to float or sink in a given liquid. If the stuff has higher density compared to that of liquid, it will sink.
If the stuff has lower density compared to the liquid, it will float.
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