Things You Thought You Knew – Force, Heat, & Speed

0:00Coming up on StarTalk, it's another Things You Thought You Knew episode. This time, we dig into force versus pressure, heat versus temperature, and speed versus acceleration. Check it out. Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now. I got more splaining to do. You got some splaining to do. Lucy? So here it is. Today I want to talk about force and pressure. Gotcha. Okay. Okay. So I'm not talking about sort of emotional pressure. Okay. That's what I'm talking about. Right. Right. You know, my job has gotten me under so much pressure.

0:50I'm talking about physics, pressure and physics force. All right. Um, by the way, another way we use those words in everyday life, we say, um, how much force are you showing on the battlefield? So that's another cultural usage of those two terms. Each of those words has a precise definition in physics. Not to mention space force. Yes, that's in there too. Okay. They don't call it space pressure. No, it's a space force. So a force is what you think it is, right? You push on something and you create a force that might set it into motion. Okay. And if it's something that doesn't move, but it's still fragile, you put enough force on it, you might break it.

1:36Nice. Yeah. Okay. So forces make things happen. And when we say happen, we mean something changes about the object. Typically it's set into motion. And Isaac Newton first wrote down an equation about this. Okay. He said force equals the mass of the object times the acceleration it'll get. If you put that force on that object Got you. Okay, so you use that formula you say well here's an object. I'm gonna put a certain amount of force and It has to be like a net force so in other words if you put a force exactly opposite mine Then the forces cancel and then there's no net forces nothing accelerates Right, so if everything is in balance you can have very high forces operating but nothing's gonna happen Right, but if there's a slight imbalance then there will be motion.

2:31And didn't long ago we talk about this like at the gym? Why is it that the person spotting for someone else does not have to be as muscle-bound as the person lifting the weights? Have you ever thought about that? Every time I go to the gym. Okay. Because somebody will say, hey, man, give me a spot, and it's always a dude who's eight times bigger than I am, and he's lifting on a building. he's actually lifting a building and he's just like and just stand there in case I drop it and he goes hey buddy can you give me a spot and I'm just like no what am I supposed to do when you're lifting like and you're struggling if you're struggling and there's a point where you can't lift it anymore you want me to come help you you want me to then take over here's why that works okay because Because if all forces are balanced, then any force will move it, no matter how small.

3:36Ooh. So watch what happens. So I'm on the bench. It's a bench press, typically, right? Because the weight is above the person's neck. Correct. And so this is dangerous. You don't need a spotter if you do a bent over a sort of rowing lift. No, because you can just drop the weight. You just drop it. It's no big deal. When it's over your windpipe, it's like, hey, Chuck, can you spot me? And I'm like, hey, man, you want to die. It's okay. You get my skinny ass to prevent you from dying. So watch. So here I am. I'm lifting and that's getting harder and harder. All right. And now there's a point where I get it halfway and I can't get it any further.

4:14And I say, Chuck, help me out here. in that moment, my upward force equals the downward force of those weights. And force on earth from gravity is called your weight. So the weight equals the force pushing up on it. If they're equal, now the thing has just stopped moving. Okay. It has stopped moving. So now you come along and say, here you go. And then you lift and you lift. You could probably use one hand to do this. You lift it back up onto the rack. Because the forces were balanced. Whereas previously, the person's force was greater than the weight of the weights. Right? And so if it's greater, I'm in control here.

4:58And I can push the thing away from Earth. Away from Earth's urges to try to bring it back. When we're in balance, then you break that tie basically and put it over the hump. That's why that works. That's very cool. Okay. So we're teaming up on the weights, basically. You're teaming up, right. And it doesn't make a difference how strong I am. I could take two fingers and just whatever little bit I'm doing, now you're winning. Provided that he's not losing that battle. Okay. If the weight is on its way down, you're going to need, it's not balanced. You have to counteract that. Right. And then put in a little more to get the thing back up to the stack.

5:41And that's when I stand over top of him and go, sorry, man, you're going to die. You sound like this has happened before. All right. So just get a sense of what forces are. Okay. That's all. And oh, so with regard to acceleration, if there's a net force, then the object's motion will continue to increase in speed. You have an acceleration. All right. So there you have it. One last thing, just in detail. If all forces are balanced, it can still be in motion. It just won't be accelerating. Right. Okay. So you can have no motion or constant motion. If there's a net force, it will accelerate. That's the point that's going on here.

6:26All right. So you're in your car and your foot is on the accelerator pedal and you're sticking to 60 miles an hour, 55 miles an hour. Well, what does it mean if your foot is on the accelerator pedal, but you're not increasing in speed? You're not accelerating. Oh, well, the force the accelerator pedal is trying to put in the car is exactly balanced by the friction of the tires on the road and the air resistance. All of that is balanced. And you're maintaining constant speed. If you want to take it out of balance, you press the pedal even harder to overcome that balance. And now you can pass the car on the right.

7:07By accelerating up to 70, you pass him, and then you slow back down again. All right. So that's what's going on with force. And everybody learns this in like physics 101, the first 10 days. Okay. So now, what is pressure? Pressure is when you have been dating for four years and she goes, what are we doing here? Okay, Chuck. Seriously, how many times can I take you home for Thanksgiving and explain to my parents that, you know, we're not ready yet? What? I mean, what is that? That's pressure. You tell me that's pressure.

7:50Okay, that's not the kind of pressure I'm talking about here. Oh, okay. Okay. Okay. That's dating pressure. How about that? Right. So we're talking about physics pressure. So pressure intimately needs force to be what it is, but it's not the same thing. Uh-oh. Okay? Okay. It's not the same thing. So if you want to find out what it is, you got to look at the equation for pressure. Okay? Oh, okay. Have you ever seen the equation for pressure? I don't think I have. All right. Let me, before I get to that, let me tell you a few things that are affected by pressure. For example, your knife set. How sharp are your knives?

8:34That is all about pressure. All about pressure. Okay. are you going to fall through the ice on that pond as you walk across it? That's all about pressure. And stupidity. And stupidity. Right. All right. So let's talk about this. Here you go. Pressure is force divided by area. Oh. Okay. And I didn't even know that equation, but that makes perfect sense. It makes perfect sense. So watch. So watch. So if I'm walking out onto a frozen pond and I don't want to fall through, if I have tiny itty bitty ass feet, then the area of the bottom of my feet is small. But what happens if you have a small number in the denominator of a fraction?

9:35the value of that goes higher. Right. So if pressure is force divided by area and that area gets smaller and smaller the pressure gets higher and you punch through that ice and you die. You need clown shoes. You need clown shoes.

9:55Get the biggest ass shoes you can find. So that force is spread over the largest area possible. Right. So when you have a big area, the force divided by a big area makes a low pressure. And so with low pressure, now you can get across the ice without sinking through. It improves your chances of not breaking the ice. Yeah. This is what snowshoes are. What are snowshoes? They're like the mountain man snow equivalent of clown shoes. Right. Right, yeah. Because the snowshoe is this big... It's like a big net. And it attaches to the bottom of your feet. And when you walk on it, your body weight is now spread over a larger area.

10:46And you don't plunge down through deep snow. You still sink a little bit, but not as much as you would have. And then you can actually walk. Have you ever seen the width of the paws of a polar bear? They're huge. Oh, my God. It's like, oh, my God. Because there's some big mofos, and they don't want to sink through the snow. Okay, they spend a lot of their time on ice, but this matters. Okay? And so, what about your knives? When you go to cut something, you apply a force. How do you make that force as effective as possible to cut? You want the lowest possible area over which you're applying that force so that you have the highest possible pressure.

11:41Okay? You get pressure for free. So what is a dull knife? You look at it under a microscope. It's all chewed up. It's flat. So your pressure, let's say you put 10 pounds of pressure on it, is spread over this long area over the length of the blade. And you try to cut something with it and you mangle the food. You have to press even harder to get it through. A perfectly sharpened blade. What's the area of a blade edge? Tell me that. The area of a sharpened blade edge. It is so tiny that even the mildest force of that knife will cut through the food. And that's why chefs are always sharpened in their knives because they want to increase the pressure on their food.

12:29Because they don't want to have to increase their force to get the pressure they want. They're reducing the area to get the pressure they want. Sweet. So this is force versus pressure. and I don't know how many people internalize this, feel it, think about it, but this distinction between force and pressure manifests everywhere, everywhere. And by the way, it's why a tornado can explode your house. Wow. Okay. You say, oh, because the wind is high. Here's what's happening. All right. It's very low pressure in the middle of a tornado. Okay? Really, really low pressure. And inside your house, you have slightly higher pressure than that tornado.

13:22Now, suppose that pressure difference is like one pound per square inch difference, let's say. Okay? It might be a little high for this example. A tenth of a pound per square inch. I don't care. A tenth of a pound. Okay? So inside the house, the air has not equilibrated with the outside of the house yet. The tornado comes. It sits on your house. Oh, my gosh. Every square inch of your wall is feeling a tenth of a pound pressing outward. So 10 square inches feels how much? A hundred. No, it's a tenth of a pound. So 10 square inches is a pound. Okay. A hundred square inches is just 10 inches by 10 inches.

14:05That's 10 pounds. Your wall is probably bigger than 10 inches by 10 inches square. You keep adding this up. And that pressure builds on top of it. You get 1 ,000 pounds of pressure. Oh, my God. That's more than the Kool-Aid guy actually exerts to get through a wall to say, oh, yeah. So what I didn't say it right. So it's thousands of pounds of total force spread across that wall. But the whole wall is only built to handle you leaning on it or to hold up the house. It's not enough to prevent the tornado from exploding your house. And all the walls blow out. Take a look at video footage of homes.

14:51They don't collapse. They're turned into matchsticks. That's matchsticks and they explode outwards. That is pressure at its most deadly. Wow. And so, you know, there you have it. Now, see, this is what I'm talking about. When I say plot twist, no one would ever think that you just talk about force and pressure and we end up right here. Right. And by the way, it's how bombs work. What is a bomb? It sets a pressure wave, high temperature expansion of the air, because there's some, an explosion is a very high temperature abrupt device, right? But it happens rapidly so that it's like a bullet firing.

15:30It's a rapid expansion of gas, which shoves the bullet out. But if it's a bomb, there's no bullet. It's just the expanding air. Sometimes you can put in shrapnel, but air will do this. And the expanding air comes out. And now you have air pressure too high on one side of the wall versus the other. And that'll blow the wall inward rather than outward. Or if the bomb is inside the house, it'll blow the house up instead of it. Right. So this is pressure on the wall spread over the area. And by the way, if all of that force were in one spot, it would just puncture a hole through the wall. Right. That's so cool.

16:04Oh, my God. So why can't we find a way? everybody's always trying to figure out a way to predict where a tornado will go which is almost impossible why not just have like a tornado airbag well you would die never mind no i was gonna say wait chuck chuck wait chuck you don't need tools to tell you where the freaking tornado is just look that's true you try to see airbags exploding oh there must be a tornado somewhere here Right. Yeah, exactly. I'm overthinking. You're overthinking that one totally, Chuck. Overthinking. Okay, Chuck, we're done there. That's pressure versus force. That's very cool.

16:49Not to mention, very, very cool song. Under pressure. Oh, yeah, yeah. Queen. Yeah, very good. Very good.

17:09Hey, this is Kevin the Sommelier, and I support StarTalk on Patreon. You're listening to StarTalk with Neil deGrasse Tyson.

17:26It's a source of no end of misconception in our world, in civilization. Oh, okay. Yeah, yeah. Yeah, so it's a big one. Okay. All right. And it's the difference between heat and temperature. They are not the same thing. Okay, so you have already. You're right. Because if you say that this is a source of misunderstanding, then I am the source. Because guess what? Heat and temperature, I mean. It's the same damn thing. That's the same thing to me. I will start off. I hate starting off this way, but I will. I'll start off defining them from the point of view of a physicist. Okay? All right. All right.

18:14So the temperature of a thing is the average kinetic energy of its vibrating molecules. Okay. All right. So you have a thing that is of a temperature. You go, you look in close. all the molecules or if they're atoms it could be atomic they're all vibrating are they writing fast they're writing slowly okay you put a thermometer in there that vibration gets communicated to the thermometer the thermometer reads a temperature is the average kinetic energy the average energy of motion of the vibrating particles the average which means a single particle has no temperature okay okay wait a minute a single a single particle that's right There is no, it doesn't meet.

19:04So temperature is a macroscopic thing that you obtain from a liquid, a solid, a gas. It doesn't matter. Okay? That's temperature. Okay. Okay? So you heat it up some more, you get higher temperature. Oh, by the way, there's a range at which they vibrate. Some vibrate slowly. Some vibrate quickly. It's the average that's the temperature. Let me say that another way. At a given temperature, there's like the average, which is where most of them are kind of vibrating. And then there's some off at the tail. Some are vibrating slowly. Some are vibrating quickly. Okay. Okay. Here's an example. Okay. Let's get water at, let's stick to Fahrenheit.

19:49Let's say we are 200 degrees. Water. No. Room temperature water. 75 to 70 degrees. Okay. Here you go. some of those water molecules are vibrating very fast, others very slowly. Okay? Right. Some of them are vibrating fast enough to escape. Right. Yes. Okay. But it's just those only at the edge, they escape. They're at the very top. At the very top. They'll escape. The rest are stuck. Stuck. Right. Okay? So now they escape. This is evaporation. Correct. Okay. And you don't have to be boiling water to evaporate the water, because the fastest moving molecules are always escaping. Okay? That's my point.

20:31That makes sense. Also, just while we're there, if you are a low-mass atom or low-mass molecule relative to high-mass molecules, your low-mass ones are vibrating even faster on average. You can split them up. The heavy ones are moving slowly. The light ones are moving quickly. The average of all of them, that's the temperature. so funny how even atoms work kind of the way uh even molecules work the way we do you know the the heavy ones kind of slow we just kind of chill oh god oh damn i gotta get about this chair i gotta cast a chair oh give me a second And you never left the room. Right. You couldn't escape.

21:17And the lighter ones are... All right. So, for example, our atmosphere has both oxygen and nitrogen in it. And the oxygen molecule weighs slightly more than the nitrogen molecule. Okay. So, on average, if you separated out the oxygen, it would be at a lower temperature than the nitrogen. But mix them together, you only get one temperature because it's a mixture. That's all I'm saying about temperature. Okay? Okay. So heat, let's go to that individual vibrating molecule and say, how much energy you got? Write down that number. Let's go to the next one. How much energy you got? Write down that number and just keep doing it for every molecule.

21:59For every molecule. Every molecule in your soup. So it's not. OK, gotcha. So the sum of all the kinetic energies of all the vibrating molecules, that's how much heat is in the thing. Gotcha. Okay. So one is an average. The other is the actual number, the sum of all the. So your cup of coffee in the morning at 210 degrees Fahrenheit. Right. is hotter than the ocean, but the ocean has more heat. Oh, snap. It's hotter than the ocean, but the ocean, because the ocean has more molecules and you're going to add up the sum. You add up the sum of all the molecules. Total molecules, okay? That's why your coffee cup, your coffee cup is not going to start a hurricane.

23:00Right. It doesn't have enough energy in the coffee cup to make that happen. And that heat is all the energy in the ocean. Oh my gosh. And that's why the ocean can start a hurricane, but your coffee can only make your morning very bad because it's spilled in your lap. Or it can speed up your digestive tract and you're stuck in the car when you got to go to... Coffee has other consequences to your life. I got about that part. That's the last time I drink coffee and get stuck in traffic. Oh. Oh, that is amazing. So now watch what happens. So now we have climate change where the world is heating. Right.

23:47And you can say, okay, how much did the air, we don't want the air to go up by two degrees Celsius, whatever, because that could trigger other changes. Well, let's check the ocean. How much did the ocean go up? The ocean went up a fourth of a degree or like a half a degree. And you're saying to yourself, that's not much. Right. Do you know how much total energy that is? Right. Oh, my gosh. That is. Oh, my gosh. That's okay. So, Chuck, that's why when you're trying to create the energy budget of a climate system, right, there's sunlight coming in and it warms the air. Was that where all the energy goes?

24:33No, no. All the whole energy that goes into the ocean and it can hang out there lurking. All right. So you could you could reduce your carbon footprint and reduce the warming of the atmosphere. Then the ocean says, I got heat. I can dump into the atmosphere and I keep doing this even after you have corrected your behavior to protect future generations. And the balance that it's actually an imbalance at this moment, the relationship between the heat that the land retains and the atmosphere and the ocean, the ocean wins every time. Right. Because of it's it's this tremendous heat reservoir. So I just wanted to distinguish the difference between heat and temperature.

25:20And there's one little thing you might not know. OK. OK. Do you know air conditioners? Right. It's like it's hot outside and it makes you cool on the inside. Yes. OK. All right. Do you ever ask how it accomplishes this? Not really. All I know is... You just turn it on. I just turn it on and it works. And from the time that I was a kid, I know that you don't leave the door open because we're not trying to cool the whole neighborhood. What the hell? You think we're trying to cool the whole neighborhood? Shut the door! Chuck, I thought you had finished your therapy on your childhood experiences, but apparently some sessions remain.

26:06So what's happening there is, okay, there is heat inside of your room. No matter what temperature your room is, as long as it's above absolute zero, there is heat there. There is heat. There is a pump that takes that heat, removes it from your air, and sticks it outside. That's why no matter the temperature outside, if you feel the air conditioner, it's hotter at the air conditioner. Why is it hotter? because it just pulled that heat from your 72 degree room temperature room that you're trying to keep cool. It pulled it out and it can reverse that. Okay. So let's reverse it. It's called reverse.

26:45It's a heat pump, a reverse heat pump in your winter. Okay. You want it to be warmer in your room than the outside. Once you switch the heat pump, your air conditioner says, okay let me take heat from this cold air out there it's 40 50 degrees i don't care let's take heat from that cold air and put it in your room and make your room hotter even hotter than it would otherwise be compared to the outside it can do that because there is heat there no matter what the temperature is as long as it's above that as long as it's above absolute zero that is okay It's clever engineering. Just think. Brilliant.

27:28Hug your favorite engineer. This is where this comes from. Brilliant. Okay, so I'm going to admit that when we started this, I was like, this guy has really dug a hole for himself this time. No way. No way this is going to be interesting.

27:47Okay. But I got to admit, this is great. Next time you're sipping a cup of coffee looking out at the ocean. Yeah. Just think to yourself. Just know that that ocean has more heat than this hot, scalding cup of coffee. And you could burn yourself with the coffee, but the hurricane won't matter to the hurricane. That's right. Wow, that is so cool, man. That is cool. All right. That's a quick one.

28:34Speed versus acceleration. I knew one day we were going to have to have this talk.