This eighth-grade science is about buoyancy and density. It is the third lesson in a sequence of four lessons on buoyancy. The lesson is 50 minutes in duration. There are 23 students in the class.

00:00:08So, good morning.
00:00:10So, kids. I think that we can start to work with enthusiasm. Let's see what we can do.
00:00:16Before we even start. What are we trying to do? What are we trying to understand or which goal to set, Kristinko?
00:00:27Buoyancy. To figure out what it depends on. We already know; who identified buoyancy?
00:00:36Archimedes. So, he'll keep an eye on us from the rear to see if we understand him correctly.
00:00:43Or, if we still have a gap in our knowledge which we can fill together.
00:00:49Let's try it now. All that I need from you is your attention.
00:00:58I see already that a mistake has been made. Now I have it here. So, we can start.
00:01:09I have prepared a stone. And how will I find the Earth's force which pulls the stone down? How can I find out? Petre?
00:01:23We measure it with a spring scale.
00:01:24We measure it with a spring scale. So, I'll try it. Let's watch carefully.
00:01:30We don't need a precise value, it's enough to watch the red and white strip. How much?
00:01:38And now let's watch what is going to happen. What do we notice? What do we notice, Adelko?
00:01:50So, gravity acted first, and after immersion the weight of the stone was reduced because buoyancy came into effect.
00:01:58So, this was very difficult for me to understand. Let's try it in parts.
00:02:04Regardless of the physics involved, what is the first thing that you observe. What is all that we can observe, Marketko?
00:02:13That the spring scale indicates less.
00:02:15It indicates less. Has anyone noticed something else? Marketko?
00:02:19The surface level of the water has risen.
00:02:20The level of the water has gone up. Excellent. Let's pay attention one more time and try to put everything together.
00:02:33We'll watch the stone and the spring scale. Are you noticing anything overall? Zitko?
00:02:42The more it is under water, the lower the gravity.
00:02:45Do you think that the gravity is less? Daso?
00:02:50The buoyancy is greater.
00:02:51The buoyancy is greater. So, exactly what is the spring scale indicating? Does it indicate the buoyancy or the difference between gravity and buoyancy?
00:03:05It indicates the difference between buoyancy and gravity.
00:03:08Buoyancy and gravity. Do you think that my hand feels the difference or not?
00:03:14Do you think that it is easier for me to hold the spring scale or is it easier now... or now? Tadeas, Jirko?
00:03:21It is easier for you to hold it while it is in the water.
00:03:24I can hold it easier. Why?
00:03:26Because the stone is buoyant, it feels much lighter.
00:03:31It helps me, right, it helps me. Look, I have two containers with water. Let's pay close attention. I'll put it into the second container.
00:03:47Kveta will come closer to be able to read it. Kveto, come. What value does the spring scale indicate now?
00:03:55The spring scale indicates one and a half.
00:03:58One and a quarter.
00:03:59One and a quarter.
00:04:00One and a quarter. I'll pull it up a bit so that you'll be able to see the whole scale, okay?
00:04:04One and a quarter.
00:04:05One and a quarter. What does it indicate now?
00:04:09It indicates one and a half.
00:04:11One and a half. Kveto, you may sit down. How come? How is it possible? Oto?
00:04:20The other water is salty.
00:04:22And the other one, which one do you mean?
00:04:23The one on the right.
00:04:24This one? So, how might I find out whether it is really salty? What? How shall I find out, Zito?
00:04:32Taste it.
00:04:33Taste it. And do you really think if we are here with the chemists that I can taste it?
00:04:37No. You can't.
00:04:38What? So, we have to rely on what we know. Please, Oto, how did you decide that the other water is salty?
00:04:47It has higher density and therefore is more buoyant.
00:04:50It is more buoyant. So, I'll try to write down what we were able to find out.
00:04:56We're able to find out that buoyancy is connected with the volume of an object.
00:05:17You will help me with the symbol. Which? For the volume of an object?
00:05:24The letter V. And with what else will it be connected? What did Ondra say, Tadeasi?
00:05:31With the density of water.
00:05:32With the density of water. It'll be water or we may use- any liquid. And help me with the symbol here?
00:05:46The symbol rho, excellent. So, the truth which we've demonstrated is that buoyancy is related to these.
00:05:58An we'll try to inquire into- to point out how big the buoyancy is.
00:06:08We have an aid for that reason. One cylinder is full, the other is empty and they fit ideally into each other. And when they fit into each other, it means-
00:06:22What does it say? What does it say, Martino?
00:06:26That they have the same volume.
00:06:28That they have the same volume. This will be very handy. This aid was made by someone who already knew the Archimedes' principle. And therefore he wanted to help us, to make it easier for us.
00:06:43I'll try it now. And we'll pay attention again.
00:06:51So. Before we do it let's get some water. Like this. We'll prepare it and I need more water.
00:07:14So, let me use this one, for example. So I'll prepare a bowl. I have a beaker in this bowl and I'll pour the water in a way that the surface will remain smooth, okay?
00:07:33The surface should be level with the edge of the beaker. Let's watch.
00:07:40Let's look. The spring scale shows three white and two red stripes. The last white one is not shown completely. Let's look again. Which force is measured, Kristinko?
00:07:55We measure gravity. How the Earth pulls on it. Let's look. I should turn that around. What else should I notice? What else should I notice? Zito?
00:08:13That the spring scale indicates a smaller value.
00:08:15The spring scale indicates less. Why, Tadeasi?
00:08:17The water overflowed.
00:08:18The water overflowed. How much of that water overflowed? How much of that water overflowed? Kristinko?
00:08:24The same amount as is the volume of that roller.
00:08:26The same as is the volume of that roller. Therefore the spring scale indicates less, because the water is helping me.
00:08:34And pay attention. If I pull on it to get the primary value, you tell me... How big of a force is in my hand?
00:08:50How big of a force is in my hand now, Martino?
00:08:54The same as the volume of the displaced water.
00:08:57Or as is? Petre?
00:09:01The buoyant force.
00:09:02How big is the buoyancy. So, now I'll use the displaced water instead of pulling with my hand. So, let me try it this way.
00:09:16And what do we expect? What do we expect if I add the displaced water now? What does the spring scale indicate, Jirko?
00:09:30It indicates the same value as before when gravity was in effect. Gravity is still affecting it, but before we didn't have it in water.
00:09:37Hmm, excellent. How big is the buoyancy? How big is the buoyancy? Kristinko?
00:09:46The buoyancy is the same as the gravitational force of the displaced water.
00:09:51Which was displaced and which we added back. And we were able to see it. Excellent.
00:09:57So, let's us look, let's us look at the formula, which Mr. Archimedes subsequently drew up.
00:10:10We've already seen how big the buoyancy is. We described that in words and now we need to mark it in the formula.
00:10:22So, the force will play a role in there. It is the buoyancy. And repeat it one more time that the buoyancy is as big as what?
00:10:36As the gravitational force of water displaced by an object.
00:10:38As the gravitational force of water displaced by an object. So, I'll try it. The mass of an object multiplied by g, which for our country is 10 Newtons per kilogram.
00:10:58Excellent. How will I continue? What now? It is the mass of an object multiplied by a g. Do you I have the object correct or not?
00:11:13Is there a mistake? Repeat it one more time? Is it a mass, gravitational force of what?
00:11:22Water should be there, you said that correctly, but my thoughts are somewhere else, therefore I wrote that incorrectly.
00:11:32I was one step ahead. What mass is it? A mass of displaced water. Okay.
00:11:45So, I would have to- you have to watch over me. Displaced water. And how can I write down that displaced water?
00:11:54How can I write that down? Can I use the mass and the volume? May I write that mass with the help of a density and a volume? Is that possible?
00:12:09What do you think? I'll help you. Look in here, mass, density, volume. I want to calculate a mass. How can I help myself, Tadeasi?
00:12:24Take the volume of an object //and multiply it by its density.
00:12:28Excellent. The volume of an object and I'll multiply it by its density. And I won't forget the g.
00:12:37So, that is the mathematical formulation of what we saw. Okay.
00:12:45Swimming is certainly connected to buoyancy. Have you ever thought of why we have to learn to swim?
00:12:59Who has a dog at home? Does anyone have a dog? So, did you have to teach your dog to swim or does he know how to? Who knows that his dog can swim?
00:13:18Vojto? Does he know how to swim?
00:13:20He knows.
00:13:21He knows. Did anyone teach him how to swim?
00:13:23Nobody did.
00:13:24Nobody did. How is it possible that the dog can swim automatically, but humans can't, Tadeasi?
00:13:30He is buoyed by the water.
00:13:32And aren't we?
00:13:35He could be self-taught.
00:13:37He could be self-taught. He knows how to do it. Now, tell me, how is this possible? Picture a dog swimming in the water.
00:13:45What sticks out of the water while he swims?
00:13:48What is sticking out of the water when a dog is swimming? Kveto?
00:13:51A head?
00:13:52A head. All of it?
00:13:54To here. Only here.
00:13:56Probably not. For us to be able to breathe, the water must be to here. What about a dog? His snout is also as low?
00:14:09He has it relatively high. Therefore the ears, eyes, and snout stick out and he is relatively deep in the water.
00:14:18Is there any reason for the dog- that only a small part of the body sticks out? Daso?
00:14:25We've said that if the object is more immersed in the water, the buoyancy is greater.
00:14:30And it is just enough for him. With regard to humans who are afraid and stick the whole head out.
00:14:37Which results in our constant improvement of swimming techniques. Okay. So.
00:14:45And what do you think? Does modeling clay float? How will I find out if it floats? How will I find out if it floats? Oto?
00:14:58Put it in the water.
00:14:59I'll throw it in the water. So, I'll throw modeling clay in the water and be able find out if it floats or not, okay?
00:15:08So, what do you think? Is it going to float?
00:15:12No, it is not.
00:15:14Whoever thinks that modeling clay floats, please, raise your hand.
00:15:19When I'll //teach it.
00:15:20//So. What Petre?
00:15:22When I'll teach it.
00:15:23If I teach it. So, you realized, what I meant by that. So, you've said, throw it in and I'll find out. So. And, now, tell me why?
00:15:35Why did that happen? Why did that happen? Why did that happen?
00:15:42Because gravity is greater than the buoyant force.
00:15:45Than the buoyancy. Excellent. So I would like to write that down.
00:15:50So, we'll indicate that, for this modeling clay, we'll find out that the force of gravity... Martino, how is it?
00:16:05Is larger or smaller?
00:16:07It is larger than //the buoyancy.
00:16:09The buoyancy. We abbreviate a buoyancy force. The force of gravity is larger.
00:16:17And, now, tell me where the modeling clay will float? What is necessary for the forces? In terms of the forces, what is required for flotation? Adelko?
00:16:26When buoyancy is greater than gravity.
00:16:28Greater, or what would be //enough for us?
00:16:30//When it's equal.
00:16:32When it's equal. So. Our goal is, to do it in such a way that gravity and buoyancy are equal in value.
00:16:47That is our goal. How can we do it? We won't tell. You'll open it and your goal is to teach the modeling clay to float.
00:17:07It overgrew-
00:17:19So, I'll try myself- you help yourselves somehow. I warmed up the clay, so I think that it will form well.
00:17:32Who is done? Who has taught the clay to float? Raise your hand?
00:17:40So, you are successful. I'll try it a little longer.
00:17:50So, in case of those whose clay floats, try to see if it can carry a load.
00:18:00(inaudible), this way down.
00:18:03Well, try it.
00:18:08I'll put it there. It should be better.
00:18:13In what break?
00:18:18What do you need that for?
00:18:21So, how are we doing?
00:18:24All right then.
00:18:25Are we done. Excellent.
00:18:29So, do it for him.
00:18:33So, okay.
00:18:34(Would you go there) after work?
00:18:42Petriku? What does it mean? So, okay?
00:18:52So, please, did everyone teach the clay how to float? Who did it? Who did it? Excellent, put your hands down.
00:19:05Who did teach the clay to float on the first try? Excellent. So, some of you did it on the second try.
00:19:16And now pay attention for a minute. It is very important.
00:19:21Please, tell me, why the clay didn't float on the first try?
00:19:27What did you do to it and yet it didn't float? Adelko?
00:19:30We had it too flat, we didn't shape it as a bowl.
00:19:33You didn't shape it into a bowl. But it had a slight bowl shape and still it didn't float. So, not every clay bowl will float.
00:19:41According to the laws of physics what was missing? According to physics what was missing, because, although it is shaped as a bowl it doesn't float?
00:19:52Considering the forces involved, what was missing?
00:19:55The force of gravity was greater than the buoyant force.
00:19:57Gravity was still greater. Please, what are we able to tell about the density of modeling clay in general?
00:20:06It's greater than water.
00:20:07It has a greater density than water. So, please, now I need your undivided attention.
00:20:15Let your boats float. I've prepared some symbolic drawings for you, which will help.
00:20:25The first drawing is a surface of the water level without clay. The second is a surface of water with the clay shaped as a ball. And now I'll put it in...
00:20:46My boat. I still don't know if it will float.
00:20:52And Tadeasi come and look. Where should I mark it? Where should I mark it?
00:21:02We'll mark it right here, okay? And what would you tell me? Is the level of the water the highest? Look precisely against it?
00:21:11It is the highest.
00:21:13It is the highest. So, let's pay attention. It is a very important finding.
00:21:17When we had clay- let me draw what we marked. I had the water and then I put- threw clay into the water.
00:21:37How should I mark it? I have to mark it a little higher. We marked this. Without the clay, with the clay.
00:21:49And the third picture, when the clay already floats, shows that the level of the water is even higher and the clay was in the shape of a boat.
00:22:08This way we write in Russian, Russian d as a g. I don't know what came over me.
00:22:16Mo-de-ling clay. Okay, what have we done? Okay. So, it should have been mentioned immediately. So.
00:22:26Please, let's look at it now. We had a volume of water. Then we threw in the clay, which didn't know how to float.
00:22:40And then we put in the clay which knows how to float. At the same time the level is the highest there.
00:22:47What does this indicate again? What did we increase in the clay?
00:22:55Its volume.
00:22:56We were increasing its volume, which it actually displaced in the water. The space will be taken up in the water.
00:23:04Looking at the forces involved we were increasing the buoyancy. How did we do it? How did we do it?
00:23:15How did we increase the volume of the clay? Adelko?
00:23:19We flattened it more.
00:23:22And formed the sphere, excellent. That is correct.
00:23:27So, we taught the modeling clay to float. We found out that we can do that in a way such that the clay takes up a greater volume of water. Excellent.
00:23:39Please, take your notebooks out. And we'll note quickly- copy the pictures involving the floating of the clay. Okay.
00:23:50Put everything aside. Petriku, you may sit somewhere else or let me give you a cloth to clean up. Okay.
00:24:05Heading: Modeling Clay. And copy down only the three pictures.
00:24:31I placed it in a way that wouldn't allow you to see through it.
00:25:29So, write it down. When you're finished, please close your notebooks.
00:25:44So, Petre, don't play with it. Put it aside. Who is done, raise you hand. Petre, you got it wet.
00:25:52It was here.
00:25:54When you work with your math tables, be careful, or get another one. Okay, Petre?
00:26:01So, let's finish.
00:26:33So, may we continue? May we continue? Let's finish the drawings and continue.
00:26:44I've prepared something for you that you should already know.
00:26:51Can you see better? Sure? I have two different things. What are those, Adelko?
00:26:59Water and oil.
00:27:00Water and oil. Please, are you surprised that the oil is on the top?
00:27:05Not really.
00:27:07No. It didn't surprise you. Describe a real situation, where, almost every day, you're exposed to oil that is on top of water. Oto?
00:27:18In soup.
00:27:19In soup. Excellent. Or while we bake meat or cook, the fat stays on the top. Tell me why, please?
00:27:28How does it know that it should stay on the top, and water down? Kristinko?
00:27:34Because the water has a greater density.
00:27:36Water has greater; water has a greater density. What kind of mixture is it? What do you remember from chemistry? Water and oil?
00:27:47It is smaller (inaudible)
00:27:52What kind? Can you recall, it was long time ago?
00:27:57Heterogeneous mixture. Correct. Do you know how to separate them? Do you know how to separate them? How could we separate them?
00:28:10In the separating funnel.
00:28:11In the separating funnel. How would I do it if I were not a chemist? If I didn't remember the separating funnel? Could I do it at least partially?
00:28:23I would for example- try to suck it out with an injection syringe.
00:28:26To collect it from the top with a syringe. We do it sometimes when we don't want the fat food. So, this way we can collect oil or fat from the top.
00:28:38Excellent. So, this is water and oil. Do you know the Czech proverb about the truth? The truth will float on the top as-
00:28:47As oil, oil on water. Excellent. So, this is water and oil.
00:28:54What will happen, what would happen, if I were to have ice and water. What is your experience? Mix ice with water? What will happen? Vojto?
00:29:18Ice floats on top.
00:29:19Ice is on top. Please, why? Why is ice on top? Marketko?
00:29:27Because is has a lower density.
00:29:28It has a lower density. What can I derive from that? Consult the physics table to confirm that ice has a lower density. Look there.
00:29:40Consult table F11 to verify.
00:30:18So, do we have it?
00:30:20Ahh, water is denser, it has 998, but at zero degrees Celsius it has 917.
00:30:29Nine hundred and seventeen kilograms per cubic meter. Please, is it good that ice, by nature, has a lower density than water in a liquid state?
00:30:41Or is it not good? Is it better for us that lakes freeze from the top, or would it be better if ice had the greater density and lakes froze from the bottom up?
00:30:55It is good that they freeze from the top.
00:30:57Is it good? Why, Tadeasi?
00:31:00Because if the ice were on the bottom then the water would be on the top, it wouldn't be flooded.
00:31:09Hmm, that's one thing, but what would happen to life in the water?
00:31:14It would die out. Excellent.
00:31:18Please, do you have any idea as to what happens to crude oil and water?
00:31:27Crude oil will float on water.
00:31:29Crude oil will float on water. I'll ask again? Is it good? Is it good that crude oil floats on water? Consult your physics table. Confirm its truth.
00:31:50We are here.
00:32:00So. What did we find in the tables? Jitko?
00:32:09Crude oil has 850 kilograms per cubic meter.
00:32:12That is correct. Is that good from an ecological point of view?
00:32:21From one side it is, because it doesn't affect the bottom of the ocean, but on the other hand when crude oil stays afloat it might destroy the beach and so.
00:32:29What is better? Both scenarios are bad when crude oil gets into our waters. But if we were to take care of it,
00:32:39Is it easier for us when the crude oil is atop the water or vice versa? Petro?
00:32:46I would say when is atop.
00:32:48When it is on the top of water. On television we often hear about ecological catastrophes.
00:32:55When a tanker has an accident in the middle of the ocean, please tell me why after all does the crude oil appear on the shores anyhow?
00:33:08It will reach the shores. Which forces are involved or what is the reason for it?
00:33:17Regardless of our effort to clean up after an accident and eliminate the presence of crude oil, why will it still reach the shore? What is the reason, Jirko?
00:33:27Sea currents and wind.
00:33:29Sea currents and wind. What else did we forget? The wind could blow in the other direction. It wouldn't help us.
00:33:36But something what is certain to get to the shore, at the same intervals, Daso?
00:33:45High tide and low tide.
00:33:46Ebb and flow. Does anyone know the reason for it?
00:33:51When the Earth tilts then water overflows. [ laughter ]
00:33:55Petriku, did you want to entertain us, please? You are not serious. So?
00:34:01Earth's gravity.
00:34:02The moon.
00:34:04It is the gravity between the Earth and the moon. Pavel is humorous today. So, excellent.
00:34:12So far we have had liquids, solid matter in liquid, and now we could experiment with substances which are not visible.
00:34:28But they still have different densities and might either stay at the bottom or not. Let's see.
00:34:38We have enough air. So let me try. This is- looks like a liquid, but once it leaves the bottle it becomes gas.
00:34:49This is lighter gas. So, what characteristic does it possess? What characteristic does it have? Tadeasi?
00:34:59It'll burn.
00:35:01It'll burn. So, when it burns, let us try, let us see if it really will burn, okay.
00:35:10So, we clean up a little. Now I'll try, to see if the gas will burn, okay. So let me try to ignite it.
00:35:30So, //it burns.
00:35:33//Once more.
00:35:35Let's try something different. Since we are discussing density, let me show you one more thing.
00:35:46I'll try to put the gaseous fluid in here- cover it and pour it in here.
00:35:51Under what circumstances will I be able to do it? When will I be able to do it? What must be the case?
00:36:05What will allow me to do it? If I'll have this- oh, this is salty water, which we might still need.
00:36:18This way. I want to do it exactly this way. What must be the case to do it? What is the rule? Petre?
00:36:32A gas must be heavier than the air.
00:36:34Gas must be heavier than air. We would say that it has greater density.
00:36:39Well, let's see if this is going to work. I'll put it in here. Now I'll try to transfer it by pouring it.
00:36:53What do you think? Now? Or a little longer? So let me try, if I was successful. Can you see that my hand is shaking? Completed successfully. Nothing is left in here.
00:37:12Please consult your physics tables to see if any inflammable gas really has- it might be a combination of propane and butane.
00:37:25Try to find out if propane really has greater density than air.
00:37:33Propane is there and-
00:37:35Try it. Look it up.
00:37:38These are different ones.
00:37:40So, Petro, what did you find?
00:37:42That propane has greater density.
00:37:45Has greater density, therefore I knew ahead of time that my experiment would be successful. If a gas were to have lower density, how would I have to do it? I would have to-
00:37:56It would dissipate.
00:37:57It would dissipate, I would have to catch it. Excellent. And we still have one more experiment ahead of us. How will we start it? Let's do it this way.
00:38:14Excellent. So, I want to show you this.
00:38:18I have a glass cup, okay. Honey, water, and oil. What do you know with certainty about the honey? What do we know about the honey? Jitko?
00:38:35It has greater density than water.
00:38:38Oil. Excellent. what do we know about oil? What do we know about oil? Sono? Not Sona.
00:38:49It has smaller volume than water and oil. I meant density.
00:38:52Density. Excellent. So we can continue to investigate. Here, I have a Lego piece and I'll throw it in. Okay. What do we observe?
00:39:10Where is the piece? Where did the piece land? Where did it land? It went through... what? Where did it land? Kristinko?
00:39:21In the oil.
00:39:24Did it land in the oil, or did it remain partially in the water as well? Lenko?
00:39:33It remained partially in the water.
00:39:34It is also partially in the water. What does it tell us about the Lego's density? What is its density in comparison with the oil? Martino?
00:39:47It has greater density. In comparison with water?
00:39:53Lower. And quite certainly its density is lower than the honey. Excellent. And I brought cork as well. Here it is. What do you think, cork?
00:40:06What do you think that cork will do? Vojto? Milane?
00:40:09It will float on oil.
00:40:10It will even float on oil. Excellent.
00:40:15Now, let's do the last problem. I am not sure what is the best way of doing it. I'll distribute colored strips of paper to you.
00:40:29You'll write numbers on them and then we'll determine the identities. So this way. And the paper strips will indicate substances.
00:40:52So our physics tables stay at hand and let's determine what each strip represents.
00:41:10Perfect. Get your pencils ready and I would be glad if you take your //excellent.
00:41:22Please start with the brown paper strip and write: 1260 cubic kilograms. Write at the edge to leave enough space for the name. Okay?
00:41:39So write down 1260 on the brown strip.
00:41:54Take the yellow strip. One point twenty-nine kilograms per cubic meter.
00:42:17The blue one, 998 kilograms per cubic meter.
00:42:31The red one, two thou- excuse me, two point one kilograms per cubic meter.
00:42:45I am missing one. The green one, the last one, 917. Find the name in your tables. What different substances are symbolized by the colored paper strips? Okay?
00:43:24Find it.
00:43:25What do you want to choose?
00:43:28This is water.
00:43:33Where should we write it, Miss?
00:43:37Excuse me?
00:43:38Where should we write it?
00:43:39Write it next to the corresponding number, okay.
00:43:44So, find it. You are acting as detectives right now.
00:43:50You'll convert it now.
00:43:53Aren't you exaggerating?
00:43:57Now we have to divide it by-
00:44:04Whoever is done with solving the problems, please raise your hand.
00:44:16It appears that everybody is finishing.
00:44:28So, excellent. Tadeasi, what did you determine that the brown strip represents?
00:44:36The brown is glycerol.
00:44:38The brown is glycerol. So I will write the name next to it. Please, somewhere... I have a little bottle. Oh, here it is. Glycerol.
00:44:47You'll find it as glycerine in cosmetic and drug stores. It is a substance widely used in the cosmetics industry.
00:44:59It is a moisturizer. So, you'll certainly find it in hand creams. So, glycerine is the same as glycerol.
00:45:09Yellow, Kristinko?
00:45:11Air. Excellent.
00:45:13Blue. We didn't have to look it up. Tadeasi?
00:45:19Water. Green, Lenko?
00:45:23No, ice.
00:45:24Green. I made a mistake; we're talking about the green strip. //Okay?
00:45:32Ice. And we are left with red- propane. I picked red because when propane burns, we have fire.
00:45:40Okay, kids, let's get on the ball! How might we organize them by density?
00:45:55Let's do it.
00:46:14Are you done?
00:46:20So, check to see if I did it correctly. Do you agree with me? Do agree with the way that I've done it? I will stop at propane.
00:46:31Please, who has a gas heater at home? Who? Excellent.
00:46:41Where is- does anybody know where the gas meter is located? The one that indicates gas usage? Where is it usually? Jirko?
00:46:51In the basement.
00:46:52In the basement. And have you ever thought of why it would be in the basement? Why not on an upper floor? Why in the basement?
00:46:58We also have a pipe with gas here. The pipes are usually yellow.
00:47:07Next to the igniter, next to the meter there are a lot of connections that must be tightly made so that gas will not escape and leak.
00:47:17Gas can escape at the connections, not from the pipe itself. So, is there any advantage to a gas leaking in the basement?
00:47:28It is dangerous in any case. But is there at least little advantage of gas escaping in the basement, rather than upstairs? Where will the gas accumulate? Petre?
00:47:38Gas will sink to the ground, because it is denser than air.
00:47:40To the ground and when we breathe we would breathe a smaller concentration of gas than we would if the gas were coming from above.
00:47:51So. The remaining problem will be your homework. I distributed pictures, which are on your desks. Please, this is a fish, and what is this?
00:48:04A bunny.
00:48:06Bunny, or rabbit? Please you will investigate what the differences are between the skeletons of fish and rabbits. Okay?
00:48:27See if you find any differences and using physics, determine why the skeleton of a rabbit is the way that it is.
00:48:40And why the fish has such small and flexible bones. Why is it this way?
00:48:48So, that will be where we pick up next week's lesson. Okay?
00:48:58So. That's all. You have a break. I will ask someone to clean up the teaching aids. Okay?
00:49:14Do you see it?