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

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Speaker 3 Coming up on Star Talk, 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.

Speaker 4 Welcome to Star Talk,

Speaker 5 Your place in the universe where science and pop culture collide.

Speaker 3 Star Talk begins right now.

Speaker 3 I got more explaining to do.

Speaker 2 You cash explain it, Toto.

Speaker 3 Lucy?

Speaker 3 So here it is. Today I want to talk about force and pressure.

Speaker 2 Gotcha.

Speaker 3 Okay.

Speaker 3 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.

Speaker 3 I'm talking about physics pressure and physics force. All right.

Speaker 3 By the way, another way we use those words in everyday life, we say,

Speaker 3 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.

Speaker 2 Not to mention space force.

Speaker 3 Yes, that's in there too. Okay.
They don't call it space pressure. No, this is space force.
So

Speaker 3 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?

Speaker 3 And if it's something that doesn't move, but it's still fragile, you put enough force on it, you might break it.

Speaker 2 Nice, yeah. Okay?

Speaker 3 So forces make things happen.

Speaker 3 And when we say happen, we mean something changes about the object. Typically, it's set into motion.
And Isaac Isaac Newton first wrote down an equation about this. Okay.

Speaker 3 He said force equals the mass of the object times the acceleration it'll get if you put that force on that object.

Speaker 2 Gotcha.

Speaker 3 Okay, so you use that formula. You say, well, here's an object.
I'm going to put a certain amount of force.

Speaker 3 And

Speaker 3 it has to be like a net force. So in other words, if you put a force exactly opposite mine,

Speaker 3 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 going to happen.

Speaker 3 But if there's a slight imbalance,

Speaker 3 then there will be motion. And didn't long ago we talk about this like at the gym?

Speaker 3 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?

Speaker 2 Every time I go to the gym.

Speaker 3 okay.

Speaker 2 No, I'm just saying. Somebody will say, hey, man, give me a spot.
And it's always a dude who's eight times bigger than I am.

Speaker 3 And he's lifting on a building.

Speaker 2 He's actually lifting a building.

Speaker 3 And just stand there in case I drop it, right?

Speaker 2 And he goes, hey, buddy, can you give me a spot? And I'm just like, no.

Speaker 2 What am I supposed to do when you're lifting like?

Speaker 3 And you're struggling.

Speaker 3 Not only if you're struggling and there's a point where you can't lift it anymore, you want me to come help you? Right.

Speaker 2 You want me to then take over.

Speaker 3 Okay, here's why that works. Okay.

Speaker 3 Because

Speaker 3 if

Speaker 3 all forces are balanced,

Speaker 3 then any force will move it,

Speaker 3 no matter how small. Ooh.

Speaker 3 So watch what happens. So I'm there.
I'm on the bench.

Speaker 3 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 do a bent over sort of rowing lift.

Speaker 2 No, because you can just drop the weight.

Speaker 3 You just drop it. It's no big deal.

Speaker 2 When it's over your windpipe, it's like, hey, Chuck, can you spot me?

Speaker 3 And I'm like, hey, man, you want to die. It's okay.

Speaker 3 You get my skinny ass to prevent you from dying.

Speaker 3 So watch. So here I am, I'm lifting.
And that's getting harder and harder. All right.

Speaker 3 And now there's a point where I get it halfway and I can't get it any further. And I say, Chuck, help me out here.

Speaker 3 In that moment my upward force equals the downward force of those weights and right force on earth from gravity is called your weight so the weight equals the force of pushing up on it if they're equal now the thing has just stopped moving wow 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

Speaker 3 because the forces were balanced whereas previously the the person's force was greater than the weight of the weights right and so if that's greater i'm in control here and i can push the thing away from earth

Speaker 3 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 yeah that's why that works That's very cool.

Speaker 2 So

Speaker 2 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

Speaker 3 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

Speaker 3 and then put in a little more to get the thing back up to

Speaker 3 the stack.

Speaker 2 And that's when I stand over top of him and go, sorry, man, you're going to die.

Speaker 3 You sound like this has happened before.

Speaker 3 All right, so just to get a sense of what forces are, okay, that's all. And

Speaker 3 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.

Speaker 3 One last thing, just in detail. If all forces are balanced,

Speaker 3 It can still be in motion. It just won't be accelerating.
All right. Okay? so you can have no motion or constant motion.
If there's a net force, it will accelerate.

Speaker 3 That's the point that's going on here. All 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.

Speaker 3 Well, what does it mean if your foot is on the accelerator pedal, but you're not increasing in speed? You're not accelerating.

Speaker 3 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.

Speaker 3 All of that is balanced and you're maintaining constant speed.

Speaker 3 If you want to take it out of balance,

Speaker 3 you press the pedal even harder to overcome that balance and now you can pass the car on the right

Speaker 3 by accelerating up to 70, you pass them and then you slow back down again.

Speaker 3 So that's what's going on with force. And everybody learns this in like physics 101, the first 10 days.

Speaker 3 Okay, so now,

Speaker 3 what is pressure?

Speaker 2 Pressure is when you have been dating for four years and she goes, what are we doing here?

Speaker 3 Okay, just

Speaker 3 chuck.

Speaker 2 Seriously, how many times can I take you home for Thanksgiving and explain to my parents

Speaker 2 that, you know, we're not ready yet?

Speaker 3 What's, I mean, what's that? Because that's pressure. You're telling me that's pressure.

Speaker 3 Okay, that's not the kind of pressure I'm talking about here. Oh, okay.

Speaker 3 Okay, okay. That's dating pressure.
How about that, right? 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.

Speaker 3 Okay.

Speaker 3 Okay. It's not the same thing.

Speaker 3 So if you want to find out

Speaker 3 what it is,

Speaker 3 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.

Speaker 3 Let me, before I get to that, let me tell you a few things that are affected by pressure. For example,

Speaker 3 your knife set.

Speaker 3 How sharp are your knives? That is all about pressure.

Speaker 3 All about pressure. Okay.
Okay?

Speaker 3 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?

Speaker 3 All right. So let's talk about this.
Here you go. Pressure

Speaker 3 is force

Speaker 3 divided by area.

Speaker 3 Oh,

Speaker 3 okay.

Speaker 2 And I didn't even know that equation, but that makes perfect sense.

Speaker 3 It makes perfect sense. So watch.
So watch.

Speaker 3 So

Speaker 3 if I'm walking out out onto a frozen pond and I don't want to fall through, if I have tiny itty-bitty ass feet,

Speaker 3 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?

Speaker 3 The 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.

Speaker 2 You need clown shoes.

Speaker 3 You need clown shoes.

Speaker 3 Get the biggest ass shoes you can find.

Speaker 3 So that force is spread over the largest area possible. Right.
So when you have a big area,

Speaker 3 the force divided by a big area makes a low pressure.

Speaker 3 And so with low pressure, now you can get across the ice without sinking through. It improves your chances of

Speaker 3 not breaking the ice. This is what snowshoes are.

Speaker 3 What are snowshoes? They're like

Speaker 3 the mountain man snow equivalent of clown shoes. All right.

Speaker 3 Because the snowshoe is this big.

Speaker 3 It's like a big net.

Speaker 3 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 and you don't plunge down through deep snow.

Speaker 3 You still sink a little bit, but not as much as you would have, and then you can actually walk.

Speaker 3 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 they're some big mofos and they don't want to sink through the snow.

Speaker 3 Okay, they spend a lot of their time on ice, but this matters. Okay?

Speaker 3 And so,

Speaker 3 what about your knives when you go to cut something you apply a force

Speaker 3 how do you make that force as effective as possible to cut

Speaker 3 you want the lowest possible area

Speaker 3 over which you're applying that force so that you have the highest possible pressure Okay, you get pressure for free. So when you so what what what is a dull knife you look at it under a microscope

Speaker 3 it's all chewed up, it's flat, it's like, 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.

Speaker 3 A perfectly sharpened blade, what's the area of a blade edge?

Speaker 3 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.

Speaker 3 And that's why chefs are always sharpening their knives, because they want to increase the pressure on their food.

Speaker 3 Because 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.

Speaker 3 So

Speaker 3 this is force versus pressure.

Speaker 3 And

Speaker 3 I don't know how many people internalize this, feel it, think about it, but this distinction between force and pressure manifests everywhere.

Speaker 3 Everywhere. And by the way, it's why

Speaker 3 a tornado can explode your house.

Speaker 3 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.

Speaker 3 And inside your house, you have slightly higher pressure than that tornado. Now, suppose that pressure difference is like

Speaker 3 one pound per square inch difference, let's say. Okay, it's 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.

Speaker 3 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.

Speaker 3 Oh my gosh, every square inch of your wall is feeling a tenth of a pound pressing outward.

Speaker 3 So 10 square inches feels how much?

Speaker 3 100. No, it's a tenth of a pound.
So 10 square inches is a pound. Okay.
100 square inches is just 10 inches by 10 inches. That's 10 pounds.

Speaker 3 Your wall is probably bigger than 10 inches by 10 inches square. You keep adding this up.

Speaker 2 And that pressure builds up.

Speaker 3 You get 1,000 pounds of pressure.

Speaker 2 Oh, my god, that's more than the Kool-Aid guy actually exerts to get through a wall to say, oh yeah.

Speaker 3 So what I didn't, I didn't say it right. So

Speaker 3 it's a thousands of pounds of total force

Speaker 3 spread across that wall, but the whole wall is only built to handle you leaning on it

Speaker 3 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.
They don't collapse. No.

Speaker 2 They're turned into matchsticks.

Speaker 3 That's matchsticks, and they explode outwards.

Speaker 3 That is pressure at its most deadly.

Speaker 3 Wow. And so, you know, there you have it.

Speaker 2 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.

Speaker 3 That's right. And by the way, it's how bombs work.
What is a bomb?

Speaker 3 It sets a pressure wave, high-temperature expansion of the air, because there's some, like an explosion is a very high temperature, abrupt device, right?

Speaker 3 So it happens, it has to happen rapidly so that it's like a bullet firing. It's a rapid expansion of gas, which shoves the bullet out.
But if it's a bomb, there's no bullet.

Speaker 3 It's just the expanding air. Right.
Sometimes you can put in shrapnel, but air will do this. And the expanding air comes out, and now you have air.

Speaker 3 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.

Speaker 3 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.

Speaker 2 Oh 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.

Speaker 2 Why not just have like a tornado airbag? Well, you would die.

Speaker 3 Never mind.

Speaker 3 No, I was going to say. Wait, Chuck, Chuck, wait, Chuck.
You don't need tools to tell you where the freaking tornado is. Just look.

Speaker 3 That's true. You try to see airbags exploding.
Oh, there must be a tornado somewhere here. Right.

Speaker 3 Yeah,

Speaker 3 exactly.

Speaker 2 I'm overthinking.

Speaker 3 You're overthinking that one totally, Chuck.

Speaker 2 Overthinking.

Speaker 3 Okay, Chuck, we're done there. That's pressure versus force.

Speaker 2 That's very cool. Not to mention very, very cool song

Speaker 2 under pressure.

Speaker 3 Oh, yeah, yeah. The queen.
Yeah, that's very good. Very good.

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Speaker 9 Hey, this is Kevin the Sommelier, and I support Star Talk on Patreon. You're listening to Star Talk with Neil deGrasse Tyson.

Speaker 3 It's a source of no end of misconception in our world, in civilization. Oh, okay.
Yeah, yeah. So it's a big one, okay? All right.
And it's the difference between heat and temperature.

Speaker 3 They are not the same thing.

Speaker 2 Okay, so you have already.

Speaker 3 you're right.

Speaker 2 Because if you say that this is a source of misunderstanding, then

Speaker 2 I am the source.

Speaker 3 Because guess what?

Speaker 3 Heat and temperature, I mean,

Speaker 3 same damn thing. It's the same thing to me.
All right, I will start off. I hate starting off this way, but I will.

Speaker 3 I'll start off defining them. from the point of view of a physicist, okay? All right.
All right. So the temperature of a thing is the average kinetic energy of its vibrating molecules.

Speaker 3 Okay.

Speaker 3 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?

Speaker 3 Are they writing slowly? Okay. You put a thermometer in there.
That vibration gets communicated to the thermometer. The thermometer reads a temperature.

Speaker 3 It is the average kinetic energy, the average energy of motion of the vibrating particles, the average, which means a single particle has no temperature.

Speaker 3 Okay.

Speaker 3 Okay. Wait a minute.
A single particle. A single particle that's vibrating.
There's no,

Speaker 3 it doesn't meet right. So temperature is a macroscopic thing that you obtain from

Speaker 3 a liquid, a solid, a gas. It doesn't matter.
Okay.

Speaker 3 That's temperature. Okay.
Okay. So you heat it up some more, you get higher temperature.
Oh, by the way, there's a range of at which they vibrate. Some vibrate slowly, some vibrate quickly.

Speaker 3 It's the average that's the temperature.

Speaker 3 And let me say that another way. At a given temperature,

Speaker 3 there's like

Speaker 3 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.

Speaker 3 Let's get water

Speaker 3 at, let's stick to Fahrenheit. Let's say we are 200 degrees.
Water, no,

Speaker 3 room temperature water, 75 degrees, 70 degrees. Okay, here you go.

Speaker 3 Some of those water molecules are vibrating very fast, others very slowly. Okay, right.
Some of them are vibrating fast enough to escape.

Speaker 2 Right, yes.

Speaker 3 Okay, but it's just those only at the end at the edge, they escape.

Speaker 2 They're at the very top, right?

Speaker 3 At the very very top. They'll escape.
The rest are stuck. Stuck, right.
Okay? So now they escape. This is evaporation.
Correct.

Speaker 3 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.
Okay. That makes sense.

Speaker 3 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.

Speaker 3 You could 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.

Speaker 2 So funny how even atoms work kind of the way

Speaker 2 even molecules work the way we do.

Speaker 3 You know,

Speaker 2 the heavy ones kind of slow.

Speaker 3 We just kind of chill.

Speaker 3 Oh, God.

Speaker 3 Oh, damn. I got

Speaker 3 about this chair. I got to get out the chair.
Oh,

Speaker 3 and you never left the room.

Speaker 2 And the lighter ones are.

Speaker 3 so.

Speaker 3 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.

Speaker 3 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.

Speaker 3 That's all I'm saying about temperature. Okay.
Okay. So heat.

Speaker 3 Let's go to that individual vibrating molecule and say how much energy you got

Speaker 3 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 for every molecule every molecule in your soup so it's not okay got you so so the sum of all the kinetic energies of all the vibrating molecules

Speaker 3 That's how much heat is in the thing. Gotcha.

Speaker 2 Okay. So one is an average.
The other is the actual number, the sum of all the

Speaker 3 so your cup of coffee in the morning at 210 degrees Fahrenheit

Speaker 3 is hotter than the ocean,

Speaker 3 but the ocean has more heat.

Speaker 3 Oh snap.

Speaker 2 It's hotter than the ocean,

Speaker 2 but the ocean, because the ocean has more molecules and you're going to add up

Speaker 3 the sum of all the molecules total molecules okay even that's why your coffee cup your coffee cup is not going to start a hurricane right it doesn't have enough energy in the coffee cup to make that happen and that and that heat that heat is

Speaker 2 going to be 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 filled in your lap.

Speaker 3 Or it can speed up your digestive tract and you're stuck in the car when you got to go to.

Speaker 3 Coffee has other consequences to your life. What about that part?

Speaker 3 That's the last time I drink coffee and get stuck in traffic. Oh,

Speaker 3 so that is a very important difference. So now watch what happens.
So, so now we have climate change where the world is heating. Right.
And you can say, Okay,

Speaker 3 how much did

Speaker 3 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?

Speaker 3 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.

Speaker 3 Do you know how much total energy that is? Oh my gosh, that is

Speaker 3 oh my gosh, that's okay.

Speaker 3 So, Chuck, that's why when you're trying to

Speaker 3 create the energy budget of a climate system, right? There's sunlight coming in and it warms the air. Is that where all the energy goes? No.

Speaker 3 No.

Speaker 3 The whole

Speaker 3 energy that goes into the ocean and it can hang out there lurking. All right.

Speaker 3 So

Speaker 3 you could reduce your carbon footprint and reduce the warming of the atmosphere.

Speaker 3 Then the ocean says, I got heat, I can dump into the atmosphere and I can keep doing this even after you have corrected your behavior to protect future generations.

Speaker 3 And the balance, 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

Speaker 3 it's this tremendous heat. reservoir.
So I just wanted to distinguish the difference between heat heat and

Speaker 3 temperature. And there's one little thing you might not know.
Okay. Okay.

Speaker 3 Do you know air conditioners, right? It's like it's hot outside and it makes you cool on the inside. Okay.
Yes. Okay.

Speaker 3 All right. Do you ever ask how it accomplishes this?

Speaker 3 Not really. All I know is.
You just turn it on. I just turn it on and it works.

Speaker 2 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.

Speaker 3 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.

Speaker 3 So what's happening there is, okay,

Speaker 3 there is heat.

Speaker 3 inside of your room,

Speaker 3 no matter what temperature your room is, as long as it's above absolute zero, there is heat there.

Speaker 3 There is a pump that takes that heat, removes it from your air, and sticks it outside.

Speaker 3 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?

Speaker 3 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.

Speaker 3 It's called reverse, it's a heat pump, a reverse heat pump. In your winter, okay?

Speaker 3 You want it to be warmer in your room than the outside. Once you switch the heat pump, your air conditioner says, okay,

Speaker 3 let me take heat from this cold air out there.

Speaker 3 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.

Speaker 3 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.

Speaker 2 That is, okay.

Speaker 3 It's cover engineering. Just think.

Speaker 3 Go hug your favorite engineer. This is where this comes from.
Brilliant.

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Speaker 2 No way.

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Speaker 2 Wow, that is so cool, man.

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Speaker 3 Speed versus acceleration.

Speaker 2 I knew one day we were going to have to have this talk.

Speaker 3 Sit down, Chuck.

Speaker 2 Chuck,

Speaker 2 I need a word with you.

Speaker 2 Son, I've been meaning to talk to you about speed versus acceleration.

Speaker 3 You're of age now where this is the time.

Speaker 2 Don't worry, there's nothing to be embarrassed about.

Speaker 3 So, there's a nice scene, nice, there's a

Speaker 3 rememberable scene in the movie Top Gun

Speaker 3 where they just came out of their out of their planes and they're holding their helmet. And what does one of them say to the other as they high-five each other?

Speaker 2 I've got the need

Speaker 3 for speed.

Speaker 3 Okay.

Speaker 3 I thought it was like,

Speaker 3 I feel the need for speed and I want to push back on that, if I may.

Speaker 2 Okay. You want to push back on the need for speed?

Speaker 3 Yes, I am. Oh, no.
Because I claim that

Speaker 3 their speed is almost irrelevant to what it is

Speaker 3 that's triggering their emotions. Really? Yeah.
Yeah.

Speaker 3 Because, for example, right now.

Speaker 3 At our latitude on Earth, the rotation of Earth is carrying us due east at 800 miles an hour.

Speaker 3 Are you saying I feel the need for speed and this is great? No, you...

Speaker 2 That may explain why I keep throwing up

Speaker 2 every time I stand up.

Speaker 3 It could be a reason why I'm volunteered.

Speaker 3 No, but see, I'm about to say that what we think of as motion sickness is not motion sickness, it's acceleration sickness.

Speaker 3 Okay. Okay, so Earth is in orbit around the Sun, 18 miles per second.

Speaker 3 All of these speeds are way faster than anything they're doing in their airplane. This is true.
So it's not really after speed.

Speaker 2 Wow, 18 miles in a second.

Speaker 3 In a second. One second.

Speaker 2 From my house, I would overshoot the Bronx.

Speaker 3 I mean,

Speaker 3 I would overshoot Brooklyn from where I am right now. You'd end up in Long Island Sound.
I would. Oh, wow.
In one second.

Speaker 3 Okay.

Speaker 3 So you live in Jersey.

Speaker 3 You cross cross the hudson river the width of manhattan all brooklyn and then you come out the other side the other side

Speaker 3 oh my god all right it's amazing so here's the thing when you are moving at constant speed

Speaker 3 your body has no idea you're moving at any speed at all

Speaker 3 okay it's only when your speed changes

Speaker 3 that you get some sense of motion.

Speaker 3 And by definition, when your speed changes it's an acceleration now in in my in physics an acceleration can be positive or negative in in in the English language we have another word for when it's negative acceleration it's just called what deceleration deceleration okay so so I might say acceleration in this in my next few minutes I mean increasing

Speaker 3 or decreasing it doesn't matter either positive or negative acceleration

Speaker 3 okay when that happens

Speaker 3 you feel it, and that's what you're reacting to. All right, by the way, think of velocity.

Speaker 3 So a velocity, a change of velocity is an acceleration.

Speaker 3 And a velocity has a direction.

Speaker 3 But suppose you're banking a turn, your direction is constantly changing.

Speaker 3 Well, if velocity has to have one direction, now I'm changing the direction. That's also an acceleration.

Speaker 3 So here's my point.

Speaker 3 When you're in a moving object, no matter its speed, if the direction or the speed changes, you are accelerating. And when you feel an acceleration, your body is going to respond.

Speaker 3 If you accelerate forward, your body will be thrown backwards.

Speaker 3 If you decelerate quickly, your body goes forwards. If you bank a turn, you lean.

Speaker 3 against the door or next to the person next to you in the front seat. So that's how you know you're accelerating because your body is

Speaker 3 responding in this way.

Speaker 3 So these folks said, I feel the need for speed, it's because they're doing barrel rolls in their plane and upside down and all the stuff they're doing. That's what they're feeling.

Speaker 3 But if they were going perfectly at Mach 1, 2, 3, 4, or 30,

Speaker 3 they wouldn't be saying, I feel the need for speed.

Speaker 3 Because that's not anything they would notice. This has been the complaint about the Lexus car.
When it first came out, the Lexus was a, you know, a luxury car and that ride was smooth.

Speaker 3 I read one commentary and it said, it's like sitting on your living room couch while you're driving your car. That sounds lovely.

Speaker 3 So nobody who feels the need for speed is buying a Lexus. They want a car that can bank turns and go from zero to 60 in whatever how many seconds you're talking about.
That's an acceleration.

Speaker 2 Yeah, but it doesn't sound good to say. I feel the need for acceleration.

Speaker 2 It's a celebration of acceleration. Now I just sound like Jesse Jackson.

Speaker 3 You know,

Speaker 3 that's what I'm saying.

Speaker 3 My man rhymes anything that comes out of his mouth. Celebration of acceleration.

Speaker 3 Keep home alive.

Speaker 3 Okay.

Speaker 3 So, so that's all I'm trying to tell you. So that's why

Speaker 3 they will give top speed when you're buying a car. They will give a top speed.

Speaker 3 But they will also give 0 to 60 or 0 to 50 in a certain amount of time. So that is the change in velocity over a certain amount of time.

Speaker 3 And so if you change velocity in less and less amount of time, your acceleration is higher and higher and higher. That's why they keep trying to drop the acceleration time.

Speaker 3 Then it's more ahead. It's more head snapping.
Now. Right.
Yeah.

Speaker 3 Now.

Speaker 3 That's why everybody loves Tesla.

Speaker 3 Oh, because

Speaker 3 it'd be true for any well-made electric car will have very high acceleration,

Speaker 3 even at low speeds. Right.
Teslas can accelerate 0 to 60 in three, four seconds. Yeah.

Speaker 3 And I've been in it, and you can feel it. It's like,

Speaker 3 yeah. Okay.
Okay. So now, watch.
Let's kick it up a notch. You ready? I don't think you're ready.
Are you seeing it? Okay. All right.
I'm seeing it. Okay.

Speaker 3 There is. Strap in.
Hold on.

Speaker 2 Because I don't want to accelerate too fast.

Speaker 3 I better strap in.

Speaker 3 Okay. So if acceleration is the rate of change of your velocity, okay, so if that if your rate changes quickly, you have high acceleration, you will feel this response all the more.
Okay, all right.

Speaker 3 If acceleration is the rate and change in your velocity,

Speaker 3 what happens when you have a rate of change of your acceleration?

Speaker 3 Oh my goodness.

Speaker 2 Let me guess.

Speaker 2 Your head explodes. Yes, well.

Speaker 3 Okay. So if you have a rate of change of acceleration, that has a term in physics.
It's called the jerk.

Speaker 3 Okay.

Speaker 3 All right.

Speaker 3 So watch.

Speaker 3 Oh, man. That's great.
Okay. So watch what happens.
You ready? Go ahead.

Speaker 3 So I'm headed towards a brick wall. I'm trying to come up with these examples on the spot.
Headed towards a brick wall. And it's, I should put on my brakes.
So you put on your brakes, okay?

Speaker 3 And while you put on your brakes, you feel yourself you're leaning into the shoulder strap, okay?

Speaker 3 When you hit the wall, your body jerks forward

Speaker 3 because you had a steady slowing down of your speed until your speed went to zero instantly.

Speaker 3 So that is a rate of change of your acceleration and then you feel a jerk.

Speaker 3 Okay,

Speaker 3 but

Speaker 3 why did we run into a wall?

Speaker 3 Okay, so the jerk is what actually

Speaker 3 does sort of musculoskeletal damage in an accident.

Speaker 3 Okay. Okay, because we can sustain an acceleration.
When they say I have 1G, 2G, those are pure constant accelerations. But if you go from 1G to 6G in an instant, your whole body snaps.

Speaker 2 Right.

Speaker 3 That's this. And so the jerk is one reverse.
And the same thing reverse. Correct.

Speaker 2 So, what you're basically saying is: jumping out of a 20-story window doesn't kill you.

Speaker 3 That's correct.

Speaker 3 It's the ground that does that.

Speaker 3 It's the ground.

Speaker 3 If there were no ground, right?

Speaker 3 Oh, man.

Speaker 3 So, so that's velocity,

Speaker 3 acceleration, and jerk. So almost every, and there's some cars they say in this car you can feel the road.
If you ever test drive like a sports car, they tell you that, right?

Speaker 3 Well, what does it mean to feel the road?

Speaker 3 Well,

Speaker 3 if the road were perfectly smooth, you wouldn't feel anything. So the fact that the road has certain bumps, the Lexus wouldn't feel those bumps because the tires are adjusting to it.

Speaker 3 But your sports car, which has, quote, rigid suspension, it is rigid enough so that you're feeling that. All right.

Speaker 3 So you and the road and the bumps and wiggles and the turns and twists on the road, you're feeling it all. Nice.
You feeling it. And so you, so this is what you like.
This is what you seek.

Speaker 3 This is what the sports enthusiast is actually after, even if they're not self-conscious of it.

Speaker 3 Because if they only wanted high speeds, you can just get on a, you know, get on a high-speed train and then you don't feel it because they're smooth.

Speaker 3 No, you want to, you want to bank the turns and feel it.

Speaker 2 That reminds me of a guy on the I was on the turnpike and a guy comes by on a motorcycle and he's already doing 80, so he had to be doing a little faster than 80 because he came by me.

Speaker 2 And then he pulls back on the throttle and pops a wheelie at 80 miles an hour

Speaker 3 and pulls off. Okay.

Speaker 3 So

Speaker 2 and I and I'm pretty sure he was like, I feel the the need for acceleration.

Speaker 3 And with the high accelerating cars, of course, a constant acceleration is a one-time thing.

Speaker 3 By the way, you either press yourself back or forward or lean one way or another, and any abrupt change in that creates this jolt. But even if

Speaker 3 you're going at zero and you need to floor it, There is the initial head snap. Okay?

Speaker 3 That's a very high moment of acceleration, but then you stays that way until you like hit the brick wall and then you're snapping another way. So anyhow, I'm just putting all this out there.

Speaker 3 In case you didn't know.

Speaker 2 So all I can say is please take Neil's word for everything he just said. Let's not try the brick wall experiment for ourselves.

Speaker 3 Okay.

Speaker 2 We're not responsible for anybody

Speaker 2 who crashes their car into a wall. All right.
Just take his word for it.

Speaker 3 All right, there it is once again, Juck. You heard it here, and I'm Neil deGrasse Tyson.
As always, keep looking up.

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