Sal talks about what mass means. He also examines the masses of real objects in grams and kilograms.
- So let's talk about the unit of measurement called the "gram." And the gram is a unit of measurement for measuring mass. Now, what is mass? Well, you might have seen in everyday language, someone might look at a feather, let me see if I can draw a good feather. So a feather like this. And say, "Hey, this is not very massive." This is not massive. And notice, you have that word "mass" in there. And then they might point to some big boulder, some big heavy boulder, let me draw a big heavy boulder. And say, "This is massive." This is massive. And so, just with those examples, you might get an intuition for what mass is. It's how much stuff there is, and it's related to weight. And I said "related to weight," not the same thing as weight, because later on when you take Physics, you will learn that mass and weight are related, but that they're not exactly the same thing. Now, if you're on a given planet, and the same distance from the center of that planet, roughly speaking, mass and weight will be strongly related to each other. Something with more mass is going to have more weight. Two things with the same mass, once again if I'm on the same planet, at the same distance from the center of the planet, they're going to have the same weight. So you'll often hear people say, "How many grams does that thing weigh?" They really should be saying, "How many grams does that thing mass," but it's okay, because as long as we're staying on earth, roughly you know, at sea level, plus or minus a little bit, or even if we're a little bit further away from sea level, mass and weight, it's not crazy to hold them a little bit interchangeable. And one way to gauge something's mass is to hold it if you can, and say, "Okay, how much does this weigh," because the more something weighs, the more its mass is going to be. The more something's mass is going to be, the more it's going to weigh, once again, assuming we're on the planet, etc., etc. When you take Physics class, you'll learn that they are different concepts that are closely related to each other. So, that's what a gram is. Now, another thing that you will see in the context of mass and measuring mass, is the kilogram. And all the kilogram is, and you're going to see the prefix "kilo-" over and over again, in your, I guess your education careers, especially when you're thinking about units. But "kilo-" is just a prefix for 1000. So that literally means 1000 grams. So instead of saying "2000 grams," I could say "2 kilograms." So with that out of the way, let's see if we can get a sense for what one gram or different numbers of grams of masses are and what might have a mass of one kilogram or 10 kilograms or 100 kilograms. And that's exactly why I got all of these pictures prepared. So if, if you want to get a sense of something that has a mass of one gram, think of a paper clip. And I have this little squiggly thing here, because that right there, that means "roughly." Every paper clip isn't going to be exactly 1 gram; it might be 1 1/2 grams, it might be small paper clips that are 1/2 gram. But it gives you the sense of what would have a mass of 1 gram. And if you want to know how much something with a mass of 1 gram weighs on the surface of the earth, well, you could just hold that paper clip and kind of judge, feel how much it's pulling down on your hand. Now, or how much the weight of it is pushing down on your hand, I guess I could say. Now, if you want to get a sense of things that are little bit more massive, a pencil is five times as massive as a paper clip. And hopefully you'll see that, if you hold five paper clips on one side and a pencil on the other side, you'll see that they roughly have the same mass, and their weight will be roughly the same. Now, a little bit more massive, you could think of a field mouse. A field mouse is equivalent to the mass of roughly 4 pencils, or 20 paperclips. You want to get even more massive, you can think of a kitten. So, a kitten after a couple of weeks, it's still a kitten now, it's still not a fully grown cat, it'll have a mass of about 200 to 300 grams. And this makes a rough amount of sense, you can imagine this thing having the same mass as maybe 10 or 15 of these little field mice. You can imagine the mouse is about that size, right over there. I could do a better job drawing a mouse. But that makes some sense. So what if we get even larger? What if we get into the ranges of, say, 1000 grams? Well, now we can start talking about kilograms. And just to be clear, when I put a "g" here, that's just an abbreviation for "gram." So this is 1 gram, 5 grams, 20 grams. And now, if you want to go to 1000 grams, you could think of something like a hard-cover textbook like this one. And they're obviously, some textbooks are huge, but the one that I'm showing here I didn't weigh or figure out the mass of this exact one. But it would have a mass of roughly 1 kilogram, or 1000 grams. So that's a kilogram right over there, 1000 grams. A fully grown cat, that's about 3 to 5 kilograms, depending on how fully grown it is or I guess how big of a cat it is. So, that's pretty interesting, a fully grown cat is more than 10 times the mass of a kitten. Actually that makes sense, most fully grown humans are more than 10 times their mass when they were very, very young, or actually roughly 10 times the mass of when they were very young, so it actually makes sense. Now, if you want to get even larger, you can go to a large ape, like a human being, in particular, this one right over here. 70 kilograms, and this is actually pretty close to my actual mass, 70 kilograms. And you get a sense of what that would mean if you're used to dealing with pounds. I weigh a little over 160 pounds, so that's about 70 kilograms. Or 1 kilogram is roughly 2.2 pounds if you are used to thinking in terms of pounds. But, once again, pounds are a unit of weight, while kilograms are a unit of mass. And then, if you want to get even heavier, you would get 10 times more massive than a human being, than a normal-sized human being, you might get to something like this cow, and this cow would have a mass of 750 kilograms. So, how many paper clips would you need to have the same mass as this cow? Well, you might be tempted to say 750, but remember this is 1 gram, this is 750 kilograms. So the mass of this cow is actually 750,000 grams. So you would need about 750,000 of these paper clips to have the same mass as this cow.