The astronauts float weightless in spaceships circling the Earth. Why did their weight actually disappear? And what, to be precise, is the meaning of microgravity?
Zvi Atzmon, Galileo
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Why did the astronauts' weight actually disappear? Photo: NASA
Ask people around you what they know about things that are thousands of kilometers away from here: What are the famous sites in Paris? What is typical of London? What is the capital of Argentina? - It is likely that you will get accurate answers from many of the respondents. Now ask them about what is hundreds of kilometers away, above our heads - on a space station or a shuttle.
So why do they have no weight?
Ask, for example, "Why are astronauts weightless?" I have a fear that in this case many will offer strange explanations, no less than the explanation that the residents of Australia must walk on their hands, because they live on the opposite side of the earth... If you do ask the people who come your way the question "Why do astronauts in a spaceship have no weight?"
I have the impression that you won't be delayed long until you get answers such as "Well, it's obvious! Because the spaceship is outside the atmosphere, in space", or "What's the question?! After all, the spaceship is far from the Earth, in a place where there is no gravity, so obviously there is no weight!" Two intermediate questions concerning these answers, which you will most likely get. First question: The moon is, for all practical purposes, devoid of an atmosphere. Does an astronaut walking on the surface of the moon not feel gravity? Does it have no weight?
Between levitation and weightlessness
Indeed, its weight is lower than its weight on the surface of the Earth (countless jokes have been told about diet flights to the Moon) - approximately 6 times lower, because the mass of the Moon is much smaller (approximately 81 times) than that of the Earth, but the very fact Saying that the astronaut's weight is lower on the moon implies that he has weight, not that he is weightless.
Howie says: There is no connection between emptiness, lack of atmosphere, and weightlessness. And more: Suppose you install here, on the surface of the Earth, a room where it is completely empty, a vacuum (almost, to the limit of the existing technology), and a person dressed in a space suit enters it. Will this person suddenly start floating? Will it suddenly become weightless?
Leap into the future
Well, no and no. It is even tempting to write: on the contrary, the lack of air actually increases its weight (including the suit), albeit to a negligible extent, due to the cancellation of the lifting force that the air normally exerts (as it exerts on a body immersed in it, according to Archimedes' law). By the way, the buoyant force of a current only works when the bodies have weight, not when they are weightless.
And the second question: suppose we live in a fictional future, where towers that are 300-350 kilometers high are a broken vision; No more skyscrapers, but actual skyscraper invaders... Let's suppose there are two identical twins, one an astronaut and the other maintaining roofs of super-towers. And lo and behold, one day one of the twins was standing on the roof of the tower (in a special suit, which is a necessity at this height), and at a short distance from him - right at his eye level - his twin passed him in a space shuttle.
Eliminating the similarity between identical twins
And the question is: Are their weights the same under these circumstances? And the answer: no and no! The scales on which the astronaut stands in the spaceship indicate the number 0, while the scales on which the roofer stands indicate a considerable weight, approximately 90% of the weight indicated by similar scales standing in the lower opening of the super tower.
If so, this section is not dedicated this time to "Galileo" readers who surely know the explanation, and the difference between gravity and gravity, between gravity and weight, but to those people who wonder why the similarity between the two identical twins was eliminated. In the previous section (issue 74) we mentioned Newton, his light explosion experiment.
Newton's law of gravity
Here, too, we will mention Newton, this time his global law of gravitation: every two bodies in the universe exert an attractive force on each other that is characteristic (proportional) to the product of the masses of the two bodies and characteristic (proportional) inversely to the square of the distance between them; This is the global gravity.
Indeed, the space station, or a shuttle in its orbit, or the roof of the futuristic super tower are "in the sky". But are they really that far? Well, less, much, much less, than Australia and Buenos Aires; Significantly less than London, Paris and Moscow. But what is decisive is: less, much less, than the distance between us, who live here, and the center of mass of the earth, a distance of approximately 6375 km.
What did the scales indicate?
Howie says: The distance of the astronaut in orbit from the center of the Earth is greater than your distance from the center of the Earth, but only by a number of percentages, let's say - 5%. So why are the astronauts in the spacecraft orbiting the Earth weightless?
Well, not the lack of air, not even a huge distance from the ISS, but the fact of the astronaut being in the orbit around the ISS. This is why the scales indicated such a noticeable difference between the two twins: one weighs 0 while the twin's weight is considerable, about 90% of his "normal" weight.
The twins experiment on the moon
Suppose we conduct an experiment similar to the twins experiment but on the moon. This time the two twins are astronauts. Before they board the spaceship that brings them to the moon, they are weighed, and each of them weighs exactly 90 kg. And lo and behold, they enter the coffee orbit very close to before the moon. One of the twins moves to the lander, and lands on the surface of the moon. He places scales on the ground and weighs himself. While he offsets the weight of the massive spacesuit he is in, he indeed discovers that his weight is now about one-sixth of his weight on Earth, about 15 kg. Now he installs an innovative ladder as tall as a tall building and climbs it. And here, right at his eye level, the spaceship passes by with his twin brother standing on scales. The weight shown by the scales in the spaceship: zero, and all this, as I remember, at a very small distance from the moon.
Gravity on the lap track
It is clear then that what causes the weightlessness in the spaceship is not the distance, but the height. Doesn't the force of attraction (gravity, gravitation) work in the orbit? - Not because, on a spacecraft moving in the orbit of the coffee by definition, the force of gravity acts; In the case of a space shuttle, estimate about 90% of the gravitational force exerted by the lander on the spacecraft before launch, and this is because the orbit of the shuttle, as well as the space station, is several hundreds of kilometers away from the lander.
In the case of the spacecraft orbiting the moon in our imaginary experiment, its value is actually equal to the value in which the spacecraft would have been attracted to the moon if it had landed on its surface, since the height in this experiment is tiny. Evidence that the spacecraft is under the influence of gravity is its orbit, the orbit around Earth (in the first case) or around the moon (in the second case). After all, had it not been affected by gravity, the spaceship would have continued - according to Newton's first law of motion - in a straight line movement, and would have escaped from the orbit of the coffee, which is a closed curved line (a circle or close to a circle).
Is this an illusion?
But if gravity acts on the spaceship, and on all its components and people, why do the astronauts inside it - or those who left it for a trip in space - feel a sense of weightlessness? Is this an illusion? Well no, it is not an illusion, and this is the evidence, that the scales - an objective measuring device - indicate 0 kg.
Another evidence of this is that in shuttles and space stations that are in the coffee orbit, experiments are carried out regarding the effect of weightlessness on processes such as combustion, crystal growth, distribution of liquids and their flow, tissue cultures of living cells, and more. And yet: in the bodies of astronauts who spend a long time in weightless conditions, there are noticeable and measurable anatomical and physiological changes, such as the weakening of the bones.
the speed of the shell
What is the reason, then, for the "real" weightlessness, despite the existence of gravity? - The reason is that the reference system is subject to acceleration, or in other words: that the spaceship for all its contents is subject to constant free fall.
Newton, it was he who conducted the first experiment in the coffee orbits of spaceships, with technology that suited his time, and in which he had an amazing advantage: a thought experiment. He depicted a cannon placed on a high mountain and firing shells in a horizontal direction. Newton knew that the projectile leaving the muzzle had a constant horizontal velocity (neglecting friction in the air; placing the cannon on a high mountain was intended to reduce this effect) and a vertical acceleration in the direction of the center of gravity.
If the earth was not a sphere
In his thought experiment he examined what would happen if the horizontal speed at which the shell was fired increased. And the answer - the horizontal distance that the shell will travel before hitting the ground will increase. And here, since Earth is a sphere, as you move away in a straight line, the ground "drops". Therefore, at a high enough speed the shell will fall due to gravity but will not hit the ground - it is doomed to a constant free fall around the Earth.
This is the orbit of coffee, the orbit of a satellite and the orbit of the moon. If it wasn't (almost) a sphere, but a thin and huge surface with the same mass, the whole story would be completely different...
free fall
What do you feel in such a situation, in a situation of constant free fall, and in general in a situation of free fall? The second giant of gravity, Albert Einstein, says this: "Once, when I was sitting in my chair in the patent office in Bern, the thought suddenly struck me: 'If a person is in free fall, he will not feel his own weight.' I would be shocked. This simple thought had a profound effect on me, and pushed me to develop the theory of gravitation."
Let's say you are in the luxurious elevator of our fictional super tower, an elevator with armchairs and a table with coffee and cake. You pick up the cup of coffee and bring it to your mouth. Suddenly a terrible malfunction occurs, the elevator disconnects and begins to fall in free fall. From the force of the surprise, your fingers let go of the cup, but it - as far as you are concerned - does not fall at all; As if he, too, was amazed and forgot that he had to fall...
Falling or not falling
It is clear that for an outside observer the elevator falls, but all the other components of the elevator fall along with it, so in relation to them it does not fall. Gravity also works (in this case its action will end in catastrophe), but within the frame of reference of the elevator, which is in free fall, the weight disappears.
And you, who decided to give up getting close in the elevator and instead got on a scale just before the malfunction to check your weight, from the moment the elevator began to fall in free fall the scales drop right below your feet, so that your body does not press the spring installed in them, and the hand therefore points to 0.
Really weightless
It is important to emphasize: from the moment the free fall begins, your body no longer presses on your feet, your stomach - on your pelvis, your head - on your neck; To say: you are truly weightless, it is not an illusion. And if you activate your leg muscles and forcefully push the spring balances, for a moment the hand will move, while you - according to Newton's third law - will be pushed upwards and float in the elevator!
The situation in which the components of the spacecraft and its people are subjected is known in the professional literature as microgravity. And here is the place to comment on the matter of nomenclature, which we deal with in this section. Two things should be noted about this nomenclature: first, "micro" here is in the sense of tiny in general (like a microscope), and not necessarily in the quantitative scientific sense - a millionth.
need to stay away
In order to reach real microgravity, that is to say - that the force of attraction (gravity) of the ISS will reach only one millionth of its gravity value we are exposed to as usual, we must move more than 6 million km from the ISS, a distance that no manned spacecraft has even come close to - 17 times and more than the distance to the moon.
But beyond that, the very concept of microgravity may mislead, because gravity is similar to gravitation, and we have already seen that the gravity in the orbit of shuttles and space stations is not at all tiny, but very noticeable. As usual, this term is used to describe a situation where the weight is negligible, but gravity is noticeable.
microgravity in the sense of weightlessness (with significant gravity) is a state that researchers in various fields long to achieve, as we have already seen, and also the teams that train astronauts to fly and function in weightless conditions (in the flight to Mars, conditions of microgravity will indeed exist).
As in fall facilities
This can be achieved above Earth, for short periods of time, in drop facilities (in Japan there is a drop facility to a depth of 490 m, which provides weightless conditions for about 10 seconds) or in special parabola flights for research and training purposes, such as in the four-engine jet planes KC-135's used by NASA are called, for quite understandable reasons given their parabolic orbits, "Vomit Comets".
During a single parabola it is possible to achieve up to 15 seconds of weightlessness between the steep ascent and the descent phase. Both in free-fall facilities and in these training planes, total weightlessness is not achieved, not even micro-weight, but a reduction to 1% of the weight, and all this - we have already seen - in full gravity. Whereas in the coffee track gravity works but there is no weight, full gravity with zero weight.
The knowledge of man and space
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