physics7

Transcriber:	started by Julia Lien, finished by Yao Yu.
Brief Bio:
Date finished:	1/27/2007
Proofreader:
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What is weight? Here you stand on a bathroom scale. Gravity is acting upon you. Force is mg. Your mass is m. The bathroom scale is pushing on you and is force F_scale. And that F_scale, which in this case, if the system is not being accelerated, is same as mg. That force from the bathroom scale on you we define as weight.

[0:59] When I stand on a bathroom scale, I could see my weight is about 165 lb. Now it maybe calibrated in Newtons, but that's, of course, very unusual. If I weigh myself on the moon where the gravitational acceleration is 6 times less, then I would weigh 6 times less. So far, so good. Now I'm going to put you in an elevator, and I'm going to accelerate you upwards, and you're standing on your bathroom scale. Acceleration is in this direction, and I will call this plus. And I will call this minus. Gravity is acting upon you, mg. And the bathroom scale is pushing on you with a F_s. That force, by definition, is weight.

[2:00] Before I write down some equations, I want you to realize that whenever, whenever you see any of my equations, g, g is always +9.8, and my signs, my minus signs take care of the direction, but G is always +9.8 or +10 if you prefer that. Okay, it's clear that if this is accelerated upwards, that F_s must be larger than mg, otherwise I cannot be accelerated. So we get Newton's second law. Fs (is in plus direction) minus mg (in this direction) equals ma, and so the bathroom scale indicates m(a+g), and I have gained weight. If this acceleration is 5 m/s² in this direction, I am 1 1/2 times my normal weight.

[3:00] If I look on the bathroom scale, that's what I see. Seeing is believing. That is my weight. If I accelerate upwards with 30 m/s² . 30+10=40. I'm 4 times my normal weight. Instead of my 165 lb, I would weigh close to 700 lb. I see that. Seeing is believing. That is my weight.

Now I'm going to put you in the elevator. Here you are. And I'm going to accelerate you down. This is now a. And just for my convenience, I call this now, the plus direction. Just for my convenience. It doesn't really matter. So now we have here mg, that is gravity acting on you, and now you have the force on the bathroom scale.

[3:59] Clearly, mg must be larger than F_s , otherwise you couldn't go being accelerated downwards. So now we write down Newton's second law and we get mg-F_s must be ma. The holds for acceleration down. And so I get F_s = m(g-a). This is one way of doing it, and you put in positive values for a. If a is 5 m/s², you get 10 - 5 = 5. Your weight is half. You've lost weight. Being accelerated down, you've lost weight. You could also have used this equation and not go through this trouble of setting up Newton's law again. You could've simply said, okay, this a is minus in this coordinate system. So you put in a -5 and a +10, you get the same answer. So you have lost weight when you accelerate downwards.

[5:01] Suppose now I cut the cable. I cut it. Then this a is 10 m/s², we round it off. We go down with 10 m/s². So g - a =0. You are now weightless. You're free-falling. You have no longer any weight. You look at the bathroom scale, and the bathroom scale will indicate 0. You're floating. Everything in the elevator is floating. If you have a glass with water you could turn it over, and the water would not fall out. It's like having the shuttle in orbits with the astronauts being weightless. There is a great similarity between the astronauts in the shuttle and the free-falling elevator; the only difference is that the elevator will crash, will kill you. In the case of the shuttle, it never hits the earth because of its high speed. We will talk about this much later when we deal with orbits and with Kepler's law.

[6:14] What exactly is free-fall? Free-fall is when the forces acting upon you are exclusively gravitational. Nothing is pushing on you. No seat is pushing on you, no string is pushing on you, nothing is pulling on you, only gravity. I will return to this weightlessness very shortly in great detail, but before I do that, I would like to address the issue "how could I determine your weight if I hang you from a string." So now instead of standing on a bathroom scale, you are here, here's a string. You might even have in the string tension meter as we have seen earlier in lectures, and you're holding desperately onto that string. Just like that.

[7:11] System is not being accelerated, gravity is mg, and so there must be tension in the string, T, which is pulling you up, which if there's no acceleration, must be mg. I read the scale and I read my weight. The scale indicates in my case 165 lb. While I'm hanging, I can see my weight. So you see, it makes very little difference whether I'm standing on a bathroom scale and read the force with which the bathroom scale pushes up on me, or whether I hang from a scale, extend a spring and read that value. It makes no difference. The tension here will indicate my weight.

[8:01] There's a complete similarity with the bathroom scale except in one case, something is pulling on me, in the other case, something is pushing on me from below.

Now let's accelerate this system upwards with an acceleration a, and I call this plus. Then of course, this T must grow, otherwise you cannot accelerated. Newton's second law: T-mg=ma. Tension in the string equals m(a+g). Ah. We've seen that before. No difference with the elevator. Accelerate the system, the tension will increase and you will see that. You would read that on the scale. Your weight has increased. You weigh more. Needless to say, of course, if you accelerate the system down, that you will weigh less. We just went through that argument.

[9:00] And if I cut the cable completely, you're going to free-fall. T will go to 0. a, you got minus 10, plus 10, equals 0. You are in free-fall, the scale reads 0. You are completely weightless.

[9:17] If we accept the idea of weight being indicated by the tension in the string, then there is a very interesting consequence of that. I have here a pin which is completely frictionless, and I have a on both sides a string, and the string has negligibly small mass. Now just assume that it is massless. And there is here an object m1, and there is here an object m2, and I'm telling you that m2 is larger than m1. So we all know what's going to happen. The system is going to accelerate in this direction. M2 will be accelerated down, and m1 will be accelerated up.

[10:08] What comes now is important that you grasp that. I claim that the tension on the left side must be the same as the tension in this string on the right side. T_Left must be T_Right. Why is that? It is because the pin is frictionless and it is because the string is massless. Take a little section of the string here, teeny weeny little section, then there's a tension on it. There's a force in this direction, and there's a force in this direction. These two could never be different because then this massless string will get an infinite acceleration. So there can never be a change in tension from this side of the string to the other. If you take a little section of the string here, there it is, teeny weeny little section. So there's tension on the string, and there's tension on the string. This one could never be larger than that because then this little piece of string would get an infinite acceleration. So because there's no friction on the pin, and because the strings are massless, only because of that, must the tension be everywhere the same. If there is friction in the pin which we will do later, that is not the case.

[11:25] Given the fact that the tension left and the tension right are the same, I must now conclude that these two objects have the same weight, because then we agree that tension is an indication of weight. So these objects have now the same weight, and some people may say, "Aw, that's a lot of nonsense. You must be kidding. If m2 is larger than m1, this must have a larger weight than that." Well, they're confusing weight with mass. It is true that m2 has a larger mass than m1, but it is equally true that the weight of these two objects is now the same according to my definition of weight.

[12:03] Let us calculate the acceleration of this system, and let's calculate the tension. And let's see what comes out. I first isolate here object number 1, and this is my object number 1. I have gravity, m1g, and I have a tension T. Not negotiable, T be better larger than m1g otherwise it will never be accelerated up and we know that it will be accelerated up. So what do we get? We get T -- I will call this plus direction, by the way -- minus m1g equals m times a. So T=m1(a+g). Hey, we've seen that one before. This one is being accelerated upwards. Notice it gains weight.

[13:00] That's the tension and this is the acceleration. I have one equation with two unknowns, so I can't solve it yet. But, there's another one, there's number two here. For number two, we have a force, m2g, and we have the tension up. This one better be larger than that one. Otherwise it wouldn't be accelerated down. Let me call this direction plus. The reason why I now switch directions and call this plus as well as this, there's a good reason for it. It's not so arbitrary anymore. I know that this acceleration is going to be a positive number because it's going in the direction. It's a given. If I call this negative, I would get here a negative acceleration for the same thing for which we get here a positive. That's a pain in the neck. I don't want to have a plus and a minus sign there, have to think about it that it means the same thing. So the moment that I've assigned, defined this the positive direction, I know that this acceleration will also come out to be the same sign as this one. So I flipped the signs there. So now, I apply Newton's Law. I get m2g - T = m2a. And so I get T (I'll write it here) = m2 (g-a).

[14:30] Two equations with two unknowns. Well that shouldn't be too hard to solve these two equations. You can immediately eliminate T, by the way, if you add this one with this one. You're ready? I call this equation 1. I call this equation 2. You immediately lose your T, and you get that the acceleration a = (m2-m1) / (m1+m2) * g. And you substitute that a into the equation and you find that the tension = 2mg / (m1+m2). This is very easy for you to verify.

[15:20] Let us look, this is m1m2. 2m1m2, I lost one m, 2m1m2. Let's look at these equations, let's scrutinize them a little, let's get some feeling for it. Rather than accepting them as being them equations. Let's first take the case that m2=m1 and I'll call that m. Notice that a becomes 0, and notice if you substitute m1m2 for m here, that you get 2m, you get mg. So T becomes mg. That is utterly obvious.

[16:04] If m1 and m2 are the same, nothing is going to happen. They're going to sit there, the acceleration will be 0, and the tension on both sides, which is always the same, we argued that, is going to be mg. Clear.

[16:18] Now we're going to make it more interesting. Suppose we make m2 much much larger than m1, and in a limiting case, we even go with m1 to 0. Let's do that. Would you see now, that if m1 goes to 0, this goes away, this goes away, a goes to g, and T goes to 0. If m1 is 0, T goes to 0. That is obvious! Because if I make m1 0, m2 goes into free fall, and if m2 goes into free fall, its weight is 0. And so the tension is 0, it's exactly what you see. And you see that the acceleration of that object is g, which it better be because it is in free fall.

[17:12] So you see, this makes sense. This is exactly consistent with your intuition. And if you wanted to make m1 much much larger than m2, and you take the limiting case for m2 to go to 0, you'll find again that a goes to g, and that T goes to 0, except that now the acceleration is not this way, but now the acceleration is this way, and now this object will go into free fall. And therefore there is no tension in the string anymore.

[17:51] m1, if I return to the case which we have there that m2 is larger than m1, m1 is being accelerated upwards. That's not negotiable. So it must have gained weight. m2 is being accelerated down, so it must have lost weight. So it's like being in an elevator; there's no difference. They each weigh the same. One loses weight, the other gains weight. They each weigh the same, and so I can make the prediction that if this is m2g, which was its original weight, and this, now, is the new weight, T, then m2g must be larger than T. m1 gains weight, so T must be larger than m1g. m2 loses weight, so T must be smaller than m2g. That's my prediction; it has to be.

[18:43] And I can show you that with some easy numbers. Let m1 be 1.1 kg, and let m2 be 1.25 kg. Frictionless system, and the string has a negligible mass. What is the acceleration a of the system? I get m2 - m1, that is 0.15, divided by the sum, which is 2.35, and that is approximately 0.064g. It's about 1/16 of the gravitational acceleration, so it's a very modest acceleration.

[19:29] What is the tension? Well, I substitute my numbers for m1 and m2 in there, you can take for g 10 if you like that, and you will find that the tension equals 1.17g. And now, look at what I predicted. They both weigh 1.17g. That's nonnegotiable. That is my definition of weight. The tension in both sides is the same. That's my definition of weight. This is their weight. This one had a weight 1.25g without being accelerated. You see, it has lost weight so it's accelerated down. This one had a weight of 1.1g. You see it has gained weight 'cause it's accelerated up. So, you see, the whole picture ties together very neatly and it's important that you look at it that way.

[20:28] I now want to return to the idea of complete weightlessness, and I want to remind you, a few lectures ago how I was swinging you at the end of a string in a vertical. I was swinging you like this, and I was swinging a bucket of water like this. And I want to return to that. I want to look at you when you are at the bottom of your circle and when you are at the very top of that circle.

[21:03] You go around your circle, which has radius R. Here's that circle. There's a string here, you're here. And there's a string here, and at some point in time you're there. And you're going around...let's assume you're going around with an angular velocity ω, and for simplicity we keep ω constant, but that's really not that important. Okay. This is point p and this is point s. Let's first look at the situation at point p. You have a mass, and so gravity acts upon you, mg. There is tension in the string, T. There must be--this is nonnegotiable--a centripetal acceleration upwards; otherwise you can never do this. Remember from the uniform circular motion. So there must be here centripetal acceleration, which is ω ²R, or if you prefer, v²/R, if v is the speed, tangential speed, at that point.

[22:18] It must be there. Let's look here. Right there, gravity is acting upon you, mg. Let's assume the string is pulling on you. Let's assume that for now. So, there's a tension. The string is pulling on you. Therefore, nonnegotiable when you make this curvature here, there must be a centripetal acceleration, and that centripetal acceleration must be ω² R. That is nonnegotiable; It has to be there. Let's now evaluate first the situation at p.

[23:01] And I will call this plus and I will call this minus. So what I get now is that T minus mg must be m times the centripetal acceleration. So T must be m(a_c+g). Hey, that looks very familiar. Looks like someone is being accelerated in an elevator, almost the same equation. If the centripetal acceleration at this point, for instance, were 10 m/sec², then you would weigh twice your normal weight. The tension here would be twice mg. If this were 5 m/sec²;, then you would be 1 and 1/2 times your weight. Let's now look at the situation at s.

[24:06] At point s, I'm going to call this plus and that minus. I'm going to find that T+mg must be ma_c. Newton's Second Law. So I find that the tension there equals m(a_c - g). Hey, very similar to what I've seen before. This object is losing weight.

[24:45]Let us take the situation that a_c is exactly 10 m/sec², and we discussed that last time when we had a bucket of water in our hands. If a_c, if the centripetal acceleration is, when it goes over the top, is 10, then this is 0. So the string has no tension. The string goes limp, and the bucket of water and you are weightless.

[25:11] If the centripetal acceleration is larger than 10, then of course, the string will be tight, there will be a force on you, and whatever comes out of here will indicate your weight. If a_c is smaller than 10, that's meaningless. The tension can never be negative. A string with negative tension has no physical meaning. What it means is that the bucket of water would never have made it to this point. If you tried to swing it up, as someone tried in the second lecture but didn't make it to that point, the bucket of water will just fall. They end up with a mess, but that's just a detail. 

[25:55] So, the bucket of water, when it is here, if the acceleration there, the centripetal acceleration, were exactly 10m/sec², then that bucket of water would be weightless. So I said earlier that when you're in free fall, all subject in free fall are weightless. It's like a spacecraft in orbits, or a elevator with a cut cable. It also means that if I jump off the table, that I'm weightless while I am in midair, so to speak. It means this tennis ball, while it is in free fall, it has no weight. Now it has weight, now the weight is higher because I'm accelerating it, and now it has no weight. The tennis ball is weightless, and I assume for now that the air drag plays no role.

[26:55] If I jump off the table, I would be weightless for about 1/2 sec. This is about 1 m. If I jump off a tower which is 100 m high, I would be weightless for 4 1/2 sec, ignoring air drag. I prefer today the 1/2 sec. I'm going to jump off this table with this water in my hand, and I'm going to tell you how I can convince you that as I jump I will indeed be weightless. Here's the bottle. There's a gravitational force on the bottle. My hands are pushing up on this bottle. My hands are being a bathroom scale. I feel in my muscles the need to push up. In fact, I might even be able to estimate the weight, playing the role of a bathroom scale. It's a gallon of water, that's about 9 lb.

[28:04] Now my own body, gravity is acting upon me. But I'm being pushed up right there. Suppose we jumped. There will be no pushing for me on the bottle anymore, no pushing there on me from the table, only gravitation will act upon us, and we'll be weightless. How can I show you that we're weightless? Well, if I don't have to use my muscle to push this bottle upwards, I might as well lower my hands a little bit during this freefall, and you will see that the bottle will just stay on my hands without my having to push up. Therefore, being the bathroom scale, I no longer have to push on it. I no longer, my muscles don't feel anything, and the bottle is therefore weightless. My bottle is weightless when we jump, I'm weightless, and even this bagel is weightless. We're all weightless during 1/2 sec.

[29:08] There's no such thing in physics as a free lunch. You have to pay your price for this 1/2 sec of weightlessness. What happens when I hit the floor? I hit the floor with a velocity in this direction, which is about 5 m/sec. You can calculate that. But a little later, I come to a stop. That means, during the impact, there must be an acceleration upwards. Otherwise, my velocity in this direction could never become 0. Therefore, I will weight more during this impact. There's an acceleration in this direction. The 5 m/sec goes to 0. If I make the assumption that it took two tenths of a sec, that's a very rough guess, this impact time, then the upwards acceleration will be 5 m/sec divided by 0.2, that is 25 m/sec². That means the acceleration upwards is 2 1/2 g. That means I will weigh 3 1/2 times more, remember it's a+g, so a is 2 1/2 g up, plus the g that we already have, that makes it 3 1/2 g. So instead of weighing 165 lb, I weigh close to 600 lb for two tenths of a second.

[30:23] So we get four phases. Right now, I'm my normal weight if I stand on the bathroom scale. I jump for half a second , weightless, hit the floor for about 2/10 of a second, maybe close to 600 lb. And then after that I will have my normal weight again. Now you're going to only have half a second to see that this bottle, as i jump, is floating above my hands. I will pull my hands off so you will see that I no longer have to push it. That means it's weightless. You ready? I'm ready. Three. Two. One. Zero. Did you see it floating above my hands? We were both weightless.

Now I've been thinking about this for a long long time. I have been thinking whether perhaps this could not be shown in a more dramatic way, perhaps a even more convincing way.

[31:26] And so I thought of the idea of floating a bathroom scale under my feet, tying it very loosely so that it wouldn't fall off when I jump, and then show you that while I'm half a second in freefall, that the bathroom scale indeed indicates zero. And don't think that I haven't tried it; I've tried it many times with many bathroom scales. I've made many jumps. There's a problem, and the problem is, the bathroom scales that you buy, that you normally get commercially, they indeed want to go to zero. It takes them a long time; they have a lot of inertia. Their response time is slow.

[32:04] But even if they make it to zero by the time you hit the floor, then immediately the weight increases because you hit the floor, and your weight comes up by 3 1/2 times so it begins to swing back and forth and it becomes completely chaotic, and you can no longer see what's happening. And it just so happens that about 6 months ago, I had dinner with Professor Dave Trooper*, and I explained to him that it's just unfortunate that you can never really show it that you jump off the table, have a bathroom scale under you, and see that weight go down to zero when you're in free fall.

[32:38] And he said, "Duck soup. I can do that." He says, "I can make you a scale, which has a response time of maybe 10 milliseconds. So when you jump off the table, in 10 milliseconds, you will see that thing go down to zero." And he delivered. He came through. He built this wonderful device which he and I are going to demonstrate to you.

[33:04] Let me first give you some reasonable light for this. And I will like to show you on the scale there what this scale that he built is indicating. Here is the scale. I have it in my hands. And on top of this scale is a little platform just like on your scale. This platform weighs four and half pounds. And you can see that. It says about four and half. And now you will say, "Hmm, I wouldn't want that kind of the bathroom scale. I mean, if I want to see my bathroom scale, I want to see a 0 before I wanna go up. I'm heavy enough all by myself, I don't wanna get another four and half pounds."

[33:51] The manufacturer has simply zeroed that scale for you. But obviously also your bathroom scale has a cover on it. Once you have seen these demonstrations, you will be able to answer for yourself why we don't zero this, why we really leave this to be four and half. That is the actual mass which is on top of the spring, but it's not really a spring, it's a pressure gauge, but think of it as a spring.

[34:16] Four and half pounds. Here we have a weight, which is a barbell weight, which is 10 lbs. Is this from one of your ??? or you're doing it yourself? Ten pounds. We'll put it on top here. What do you see? Roughly 14 1/2 pounds. Alright. We're going to tape it down.

[34:51] There we go. And we're going to drop it, from about 1 1/2 meters, and we drop it in here, it's well-cushioned, 'cause we don't want it to break this beautiful device. When we drop it, the response is so fast that you will see indeed that pointer will go to 0. But keep in mind when it hits the cushion, that the weight will go up. For now I want you to concentrate only on the thing going to 0, and not what comes later.

[35:26] We will deal with that within a minute. Ok. 14 and a half pounds. You know why that thing is actually jiggling back and forth? I can't hold it exactly still. And so, I slightly accelerate it upwards and downwards, and when I accelerate it slightly upwards, it weighs a little more. And when I accelerate it downwards, it weighs less. It's interesting. You can see I'm nervous. That's my nervous-tension-meter there. Ok. We're ready? Look at -- and don't look at me now! -- look at that pointer. Three, two, one, zero!

[36:08] Did you see it go to zero? All the way to zero. Now comes something even more remarkable. He said to me, I can also make the students see the response on a time scale of about fraction of a second. By the way, this is the hero who made all of this stuff. He's fantastic. He can show you the weight on an electronic scale, and this weight you will see as a function of time.

[36:55] I'll put the 10 lbs back on again. Tape it a little tighter. And so the level that you see now is 14 1/2 lbs. This is 14 1/2 lbs, and this is zero. This mark is zero. I'm going to hold it in my hands, and notice if I can hold it still, you're back to your 14 1/2 lbs. Now I'm going to drop it. You'll see it go down to a 0, it will hit the floor, the cushion, it will get an acceleration upwards, it will become way heavier than it was before, and then it will even be bounced back up in the air, and goes again into free fall. We will freeze that for you, and you will be able, we will be able to analyze it then, after it all happens.

[37:55] So, 14 1/2 lbs. Three, two, one, zero. And now Professor Troop? is freezing it for you. Now look at this. Look at this incredible picture. This is truly an eye-opener for me when I saw it. The physics in here is unbelievable. Here is your 14 1/2 pounds. Tick marks from here to here are half a second. It was half a second in free fall. And it goes to zero at no weight. Now it hits the floor, the cushion, and its weight goes up in something like a tenth of a second. Look, this is about... one, two, three, it's about three and a half times its weight now.

[38:39] So the 14 1/2 has to be multiplied by 3 1/2 or 4, which is exactly what we predicted, that it would be much higher. But now it being, it bounces off, because it's a very nice cushion, it throws it back up, so it goes back into the air, so it goes immediately to weightlessness again. And then it oscillates back and forth, and then here, you would expect that this level 14 1/2 pounds would be the same as this. And the only reason why that's not the case: it is a little cable that fell with it, which is pushing a little bit up, on the upper disc that is there, so it's making it a little lighter. Isn't it incredible? You see here in front of you the weightlessness, and you see the extra weight when it hits, and again followed by weightlessness. Dave, A+, you passed the course.

[39:30] There is a great interest in doing experiments under weightless conditions. NASA was very interested in it, and if you would jump a hundred meters up in the sky, you would only be 9 seconds up, you wouldn't even be weightless because of air drag. However if you could jump up way near the top of the atmosphere, where the air drag is negligible, then, you would be weightless, for quite some time, and that is what people have been doing for the past few decades.

[40:06] Professor Young and Professor Oman here at the aeronautics department have done what they call Zero Gravity experiments, from airplanes, and I will explain that in detail. But first I want you to appreciate that Zero Gravity is a complete misnomer. Zero weight, yes. Zero gravity, no. If you have an airplane, anywhere near earth, flying, whether the engines are on, or whether the engines are off, or whether there's free fall, it doesn't matter. There is never gravity. There is always gravity.

[40:37] Thank goodness. But, if you're in free fall, indeed, there is no weight. Apart from that, they call them Zero Gravity experiments, and why not. Maybe it sells better. They fly an airplane, which is the KC-135, and they do these experiments at an altitude of about 30,000 ft. If I can clean this as best as I can... the plane comes in at one point in time at an angle of about 45 degrees. There's nothing special about that 45 degrees. It's just the way it's done. You have to also think of the convenience for the passengers. The speed is then about 425 miles per hour (mph). So the horizontal component is about 300 mph, and the vertical component is also 300.

[41:48] The air drag is very little, let's assume for the sake of the argument, that the engines are cut, and the plane goes into free fall. It's no different from this tennis ball. It's the same thing. You're going to see a parabola. And so this plane is going to free fall and comes back to this level. And let's analyze this arc, this parabola. Right here at the top, clearly, there will still be 300 meters per second (Note: should be 300 mph) in absence of any air drag. You should be able to calculate with all the tools that you have available how high this goes from this level. In other words, what is the time that the velocity in the y direction comes to a zero? You can calculate that, and then you know how much it has traveled. Very crude numbers, this is about 900 meters.

[42:43] And it will take about 15 seconds to reach this point, so it will take about 30 seconds to go from here to here. And in those 30 seconds the horizontal displacement is about 3 1/2 kilometers. And all these numbers you should be able to confirm. Right here, the engines are restarted. During this free fall, everyone in the airplane is weightless, including the airplane itself. Now the engines start, and the engine is sort of going, the plane is going to pull up, goes into this phase, and then the plane flies horizontally for a while.

[43:23] During this phase as we just discussed, it's like hitting the floor, you need an acceleration in this direction, there will be weight increase, so there is here an acceleration upwards. And during this time, very roughly, people have about twice the weight, and then here they have again normal weight, and then the plane pulls up again, and here it goes and repeats the whole thing again, going into free fall. So again here, people have more than their normal weight. Zero weight, more than normal weight, normal weight, more than normal weight, free fall. And the whole cycle takes about 90 seconds.

[44:13] You can imagine that it's very important when you're here in free fall, when you have no weight, that when you weight comes back, and your weight doubles and Professor Oman told me, that this change from zero to twice your weight takes less than a second. That you better know where your feet are, and where your head is. Because if your head is down, and your all of a sudden double your weight, you crush your skull. So you have to be sure that you're standing straight up in the plane when your weight begins to double, and we will see that very shortly how that works.

[44:48] I want to show you first some slides from these experiments. So here you see the situation that we just described. Let us start here. That is where I started with you. The plane turns the engines off. This is the parabola. Here the engines are restarted. This is the free fall periods. This is about 30 seconds. The engine is  restarted, and during this time there's an acceleration upwards, and they call it "2 g peak". Well, they really mean 1 g, what they really mean that my weight doubles, they call that 2 g. But of course they call this 0 g, which is equally incorrect. It's not 0 g, you have no weight! This is weightless, here your weight is doubled. Here your weight is normal, here your weight roughly doubles, and you're going into another free fall period.

[45:43] And the cycle from here to here is about 90 seconds. Now the irony has it, that the reason why these flights are done, is to study motion sickness under weightless conditions. Astronauts were complaining about motion sickness. And so Professor Young and Oman have done lots and lots of experiment with airplanes and later also in the shuttles to study this motion sickness.

[46:08] I find it rather ironic, because if you and I were part of these experiments, we would get terribly sick because of the experiments! Just imagine that you go from weightlessness into twice your weight back to weightlessness, we will be puking all day! How can you study people who are sick, how can you study the sickness due to sickness? Well, they must have found a way. They do this about 50 times per day. And now I want to show you some real data which were kindly given to me by Professor Young, where you'll see them actually in the plane.

[46:51] I believe that I have to put this on one. And... start the... can you turn off this light projector? So here you see them in the plane. They're not weightless. They're climbing up. I think this is Professor Young. The guy lying on the floor must be a bit tired. The light will shortly go on, and when the light goes on, that's an indication that the weightlessness is coming up. Already went on. I must've missed it, I wasn't looking. And there they are going into weightlessness.

[47:42] Here this person is upside down here, you better get straightened up before your weight doubles! 'Cause you'll crush into the floor!

[48:02] And now it takes 60 seconds because the whole cycle is 90 seconds, and in these 60 seconds, they get ready for the next free fall, for the next weightlessness. And you will see very shortly the light will go on again, and that will tell them that the weightlessness is coming up and then they will be weightless for another 30 seconds.

[48:33] The sound that you hear is obviously the engines of the plane.

[48:42] There you go. Light goes on. They get a warning, they take their headphones off. Everything becomes weightless. They may not like that, and so they put the headphones in a secure place. You see that here, Professor Young takes his off. And there they go again!

[49:02] Swimming in midair.

[49:08] 30 seconds weightless.

[49:17] And the plane in which this happens, yeah, these things happen. I'd like to show you a last slide of the plane, that they do these experiments from. This is the plane. While this is in free fall, about a 45 degrees angle, and these people have done a tremendous job indeed in making a major contribution to the air sickness ???.

Alright. See you Friday!

Comments:

From glin - 1/27/07 10:42 PM

From welkin25 - 1/27/07 10:01 PM