Barycentric balls in space – classroom demonstration video, VP07b
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Barycentric balls in space – classroom demonstration video, VP07b


Hello! I’m Samantha Cristoforetti. I’m an astronaut on board of the International Space
Station where I have been living, working and conducting research
for several months now. I would like to introduce to you today
different objects with different masses. One is me. And then I’d like to show you these two baseball balls. They’re absolutely
identical. They are very precious and belong to my
crewmate Terry and he was kind enough to let me borrow them. This ball, which is somewhat similar in size, a little
bit smaller, but as we will see later has a much
smaller mass. And a knitting needle. All those objects, including me, appear to float. We often
say that all objects are weightless in space but
that’s not mean that there is no gravity. Let’s take a look at
this equation. This equation allows us to determine the gravitational pull on the surface of
a planet. So let’s see, we have big G. It is a constant, often refered to as
Newton’s constant. Although actually it was determined
seventy years after Isaac Newton’s death. By Henry Cavendish. Then we have big M. Big M is the mass the Earth in kilograms. So that would be around six million
million million million kilograms. And big R is the average radius of the
Earth in metres That’s about six million 370 thousand metres. Introducing numbers, we determine small g as being equal to nine decimal 81 and that can be expressed in Newtons per kilogram
and it basically represents the amount of weight per kilogram of an object in that particular point in the
gravitational field. Or we can also express it, and that’s equivalent, as
metres per square seconds. And that is the acceleration of
a free falling object in the gravitational field. You can make your own calculations for
the International Space Station. Just add to the radius of the Earth the altitude of the ISS above the surface of the Earth. Which is about 400 thousand metres. If we do that we get the value for small g roughly equal to 8 decimal 7. So on the surface of the Earth we had nine
decimal 81. Which is about 10 percent bigger. So the force of gravity, the pull gravity here upon the International Space Station is
indeed a little bit smaller. But not that much. So if there is the force of gravity up here, why am I floating? To explain that we have to introduce the
concept of free fall. So let’s use this model of the Earth. And let’s enlist the help of a friend, Paxi.
You might know her. Paxi, hello Paxi, is with me on the International Space Station. So
for the purpose of our little demonstration let’s assume
that Paxi is the Space Station. We have calculated a short time ago that the gravitational pull of the Earth up here at 400 kilometers altitude is only about
10 percent smaller than it is on Earth. So if the Earth is attracting us why don’t we, Paxi, me, the entire Space Station, why
don’t we just crash on to the Earth? Well the thing is that we also have a huge velocity vector. We are flying at about 8 km/s and that means that if there
wasn’t a gravitational pull of the Earth, we would just
keep going straight and get lost. However, because the Earth does attract us, the Earth curves our trajectory so that we keep flying
around it. In a way we can say that our trajectory, the curvature of our trajectory, matches the
curvature of the Earth. So Paxi, me, the Space Station, we are all constantly falling towards Earth. But in a way the surface of the Earth is curving away from us. So that we never crash onto it. Objects
of different mass, like let’s say me and the knitting needle are accelerated equally in a gravitation field in the absence of air resistance. This was
demonstrated nicely on the Moon in 1971 when Apollo 15
commander David Scott dropped a feather and a hammer on the Moon and they were both equally accelerated
towards the centre of the Moon and hit the surface at the same time. Also, if we could set up a controlled experiment
and had a long time to observe would could notice that all objects with mass are attracted to each other. This
knitting nee dle to me, and me to the knitting needle. How long do you think roughly it would
take for these two objects to come together due to their
gravitational attraction? So despite objects in free fall being weightless their mass does have an
effect. That effect is called inertia. Inertia is the property of objects to continue in their existing state of rest or uniform motion in a straight line unless
their status changed by an external force. And mass is a measure of an object’s inertia. So let’s see what the effects of inertia will be if we put these objects together. These three objects are now linked
together and they act as one system. Let’s say I decide to push down. And push down on one of the baseballs. The system translates down. But it also starts to rotate. And it
rotates around its centre of mass. It is like the balance point
of the system. And in this case it’s the middle point, Because these are equal baseballs. So
let’s say I pushdown on the centre of mass. In that case the whole system translates
down, but it does not start to rotate. The centre of mass is also called bary centre. And an example of a system like this
among cosmic objects is the asteroid
ninety Antiope. Now let’s see what happens if we replace one of these baseballs with the ball I introduced to you earlier today And I told you that he has a smaller mass. And I can feel that because I can feel but he has a smaller
inertia but you can’t feel it, so I want to show it
to you. So we attach it in place of the baseball. Ok, now we have a new system. A system that
is not symmetric because the baseball is more massive
than this white ball. How can we see that? Well let’s say that I want to push
down and I’m going to push down on this lighter white ball. It still rotates. But you probably noticed it does not rotate anymore around geometrical centre. This
is not the centre the of mass anymore. In fact let
me try and push down on the geometrical centre like we’ve done
before. I induce a rotation because that is not the centre of mass in this case. So let’s see if we can find the centre of mass of this system. Just by
looking at the way rotated, I would say it’s probably around here. Pretty close Examples of such systems in cosmic objects are
for example the Pluto-Charon system, but also our
Earth-Moon system. And in that case the centre of mass is
about thousand seven hundred kilometers beneath the surface of the Earth. So while these objects are weightless and I
am weightless mass still exists and it causes many interesting physical phenomena that
make space research and science so fascinating. So I really hope you enjoyed watching
this little demonstration, and the next time you watch the
International Space Station flying over our heads think of the astronauts up there, constantly falling.

About James Carlton

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100 thoughts on “Barycentric balls in space – classroom demonstration video, VP07b

  1. wow, now all those who faithfully say mass and weight are just two ways of representation should see this.

  2. If there is some gravity at that distance above the surface of the earth and it requires a large centrifugal force to maintain zero gravity what would happen if the ISS were geo-stationary?

  3. I remember when I first started understanding the concept of gravity and everything that works in and out of it. The fact that the Space Station is always falling but never landing blew my 5th grade mind and it got crazier for me when there were people out there who knew exactly how to launch and assemble the Space Station… in space.

  4. I've watched a few videos explaining how the ISS orbits around the Earth. But the demonstration with the 2 balls and a rod, made it so much easier to understand.

    I've only studied science as far as high school, but even after all these years, whenever I watch videos like this, I still feel like a kid who's just learned something about the world.

  5. Samantha, hai fatto una belli
    ssima lezione…. La Nasa sta studiando gli effetti sul corpo umano delle fasce di van Allen….. Ora voi state in un orbita sotto queste fasce….. Ma queste fasce sono state mai attraversate??!!?

  6. Big G is called the Universal Gravitational Constant where I'm from. Heard it being called the Newton's constant for the first time in my life.

  7. wow.. very educational. Looks like she has excellent teaching skills too. Overall great video 🙂

  8. What about the centripetal acceleration on the astronaut due to the rotation of the ISS?.. they shouldn't be weightless.. I make it 8.68158 m/s squared..

  9. There is interest in finding the natural period a cantilever column having a mass at one end: The other end stayed against rotation and the vertical linear cog then deformed for the initial lateral displacement the mass at the end. The restoring event creating the oscillators harmonic motion. The comparison is of interest the natural period in weightlessness verses the natural period on the planet.

  10. The interest is in proving a portion in physic for Professors Einstein's theory in relativity. The effect of gravity is in interest in Structural Numerical Analysis in Building Design.

  11. this is f***** awesome!

    basic physics straight from where it belongs, space!

    so proud of being a fellow Italian, Samantha!

  12. She gave the equation and the general formula for Gravity Force, which is interesting and informative indeed for young children, but she forgot to define and give the name and the equation (which is pretty simple indeed, available on high school books for children) of the second important Force that allows the ISS to orbit so neat earth without falling, and without leaving earth orbit and getting lost in space. The control of this Force is the real secrete of ISS, but there is no mention about that. It seems more important for her to show how round and blue is the earth.
    Very poor video. Very incomplete explanation, also with the help of useless and stupid computer generated images.
    My opinion: negative.

  13. Cmon why I never see smart videos from smart people? Always talking about the things everybody know anyway. I see it is made keeping like less than 5 year old children in mind but any parent in the world could explain these things to their children. Im just a little confused about the point of this video.

  14. At 3.14 CGI is exposed. It is all a load of fake brainwashing nonsense. Please keep it real or admit this is just a show for children.

  15. That diagram showing the height if ISS is rather inaccurate. The earths diameter is about 55mm on my screen, and the orbit of the ISS is about 70mm diameter. On scale, the depicted orbit is 1700 km above the earths surface. But I am really nitpicking here, but sometimes it is interesting to consider just how close ISS it to the ground.

  16. If you have finish the experiment with the gas balloons maybe you can asholete a space cabin to fill it with oxygen hydrogen and ather staff that the atmosfaire has and try to change the temperature off the room up or down with a chance for make it white moisture or liquidated?

  17. (I'm personally more attracted to crochet hooks than to knitting needles; but I guess I can't explain that with physics…? 🙂 )

  18. LOL 6:46 Pareidolia, Two grumpy roids In an endless lonely attraction around each other, Long past enthused at a chance to encounter, they are apart, sour faces gazing not on to each other, and there is nothing they can do.

  19. Why did she say "when you think of the iss flying over our heads" not your heads? and up there not up here if she is up there? Weird…..

  20. Hier um die Ecke werden Lebensmittel vom Tafel oder Restware verschenkt. Ist eine tolle Sache und wird gerne von den Menschen angenommen!
    Einen wunderschönen Nikolaustag wünsche ich Euch allen und hoffe, dass Eure Sachen gut angekommen sind?! Und die lieben Mäuse…..Mensch, behandelt sie gut und sie verdienen dann die beste Zeit im Rentenalter! Beste Käsesorten, eigenes Zuhause, uneingeschränkte Freiheit und eine "Rund um die Uhr" Versicherung!

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