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Physics library
Course: Physics library > Unit 4
Lesson 3: Newton's law of gravitation- Introduction to gravity
- Mass and weight clarification
- Gravity for astronauts in orbit
- Would a brick or feather fall faster?
- Acceleration due to gravity at the space station
- Space station speed in orbit
- Introduction to Newton's law of gravitation
- Gravitation (part 2)
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Gravity for astronauts in orbit
Why do astronauts appear weightless despite being near the Earth? Created by Sal Khan.
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- ok.so if the space shuttle is moving super fast than how come those astronauts don't feel any thing when they go out of the ship for what ever reason they show it like they are motionless you know like floating away why don't they move downwards to the earth?(106 votes)
- When they step (or float) out of the shuttle, the astronauts are still moving at the same speed as the shuttle. Since there's nothing to slow them down, they continue to move at the same speed and direction as the shuttle. If someone were to jump out of a moving vehicle (don't try this at home!), the person would, just for an instant, be moving at the same exact speed as the vehicle. However, friction from the air around you—and the asphalt below—would quickly slow a person down. Because there's nothing to slow the astronauts down, they continue to move at the same speed as the shuttle.(228 votes)
- Sorry still didn't get it. Sal says, "The astronauts are going so fast, they keep missing the earth".
What is that supposed to mean? How can they "miss" the earth? And if they do miss it at a point, why do they keep orbiting?
Also, is this centripetal force? Then why doesn't the shuttle fly towards the centre of the earth?
I guess I can also ask the same for comets, planets etc.(19 votes) - My mass is what the stuff im having in my body. if i get into space station i become free from gravity. now there, if i wish to measure my mass or weight, what is the method of doing it.(7 votes)
- Your mass is a measure of how difficult you are to accelerate given a certain amount of force (it can be described as the proportionality constant that relates these two vectors). Like this second definition implies, your mass is constant. Your weight, on the other hand, is the amount of force exerted upon you due to acceleration by a gravitational field. As gravitational acceleration varies depending on the strength of the field (you weight less on the moon or at the Earth's equator), this value is not constant. In space, assuming no gravitational field--or one that could be approximated as zero--you would be weightless. You would still have mass however that can be measured in space using what is called an "inertial balance." These devices use the oscillations of a spring with a known spring constant to determine the mass of an attached object and work in microgravity scenarios.(20 votes)
- Why don't the astronauts fall while spacewalking? I'm guessing it's beacause they are attached to the space station which is moving so they move too. But if they were not attached to it, would they just fall on the surface of the Earth?(4 votes)
- They do fall. In fact, both the astronauts and the space station and all other things in orbit are constantly falling in exactly the same way as a rock falls to the ground when you drop it. We say that things in orbit are in a free fall. The reason why they don't eventually hit the ground is that while they are falling downwards, they are also moving very quickly forwards.
The Earth is round. This means that the ground is curving away from you in every direction. The most visible effect this has is the horizon, which is the distance where the ground has curved so much away from you that you can't see it anymore. The fact that the Earth is curved also means that if you were to move forwards in a perfectly straight line, the ground would drop away from your feet. The faster you move, the faster the ground would drop. At a certain speed, the ground would be dropping away as fast as you would fall towards it. This means that if you move at this speed, even though you are constantly falling towards the ground, you never hit it, since the ground is curving away from you at the same rate. You are falling down, but by the time you have fallen to where the ground was, you have moved so far that the ground is still just as far below you. This is what we call orbiting.
Since you are in a free fall when you are in orbit (and you are also generally above the atmosphere, since that would slow you down enough to hit the ground pretty quickly) there is no force that is pushing on you like the ground pushes on you on the surface. This makes it seem like there is no gravity, but in fact the force of gravity is almost as strong at the space station as it is on the ground. It is just that instead of being canceled out by the force from your shoes or your chair, it is being canceled out by the centrifugal force of your orbit around the Earth. Since both the astronauts and the space station are moving with the same (really fast) velocity, they are at rest relative to each other, and the astronauts can float around the station on their space walks. Constantly falling, but never hitting the ground.(19 votes)
- If they have so high velocity so that they go around the earth that means that we cannot reach the moon.Because when ever we are going little far from earth (let we go little far than setalites) our shuttle would be in free fall and just moving round and round the earth.(7 votes)
- You are right - the space station cannot reach the moon.
When you want to reach the moon, you have to go faster, and on a path that is not a circular, orbital path.(5 votes)
- i cant understand Why do astronauts appear weightless despite being near the Earth?(5 votes)
- Imagine there is no atmosphere in the Earth. You're superman and are throwing rocks horizontally. When you throw the rock really really fast, it is possible for it to fall at the same rate the Earth is curving, which means it will stay rotating around the Earth forever.
This is the same principle that allows satellites and astronauts to stay in orbit. They are falling all the time, but missing the Earth every time. It looks like there is no gravity for them because everything around them is also falling with them.(7 votes)
- Is there any specifically measured distance between the earth and space that any spacecraft or body do not feel gravity?(4 votes)
- the gravitational field of the Earth extends infinitely throughout space but its intensity is so less that it doesn't effects bodies outside a certain limit. just like Andrew said, the strength of the gravitational field declines with the square of the distance which is known as the inverse square law;
1/x^2(7 votes)
- If the astronauts are near the earth and are experiencing gravity to keep them in orbit, why would they float, they should just be sitting on their seats and going around the earth, why is the gravity affecting the space shuttle- keeping it in orbit but not affecting the astronauts inside it and keeping them down in their seats?(3 votes)
- They are indeed going around the earth. As a part of the shuttle-astronaut system, they are acted on by gravity and they go around the earth.
What you are perhaps confusing about, is the force necessary to keep them in their seats. This must be a force that pulls them towards the seat while the seat does not concomitantly move downwards (otherwise they would not be able to sit in the seat). This is sort of a force acting on the astronaut relative (in a fuzzy way) to the seat, which gravity cannot provide since it acts on both the shuttle and the astronaut.(3 votes)
- Does time affect gravity? If it does, is time traveling through space time possible?(2 votes)
- More like the presence of a gravitational field has an influence on time.(2 votes)
- What creates mass and gravitational pull? As in, if we pulled a bunch of space junk together and created a planet-sized hunk of waste, would that have gravitational pull, and, if not, why wouldn’t it?(2 votes)
- From a Newtonian perspective, mass creates gravity. From Einstien's perspective, mass, energy, and momentum create a manifestation of curved spacetime, which we call gravity.
If we pulled a bunck of space junk and created an artificial planet, it woudl still have that gravitional our pull. This is because even you, your computer, etc. has gravity...but too miniscule to make a noticible difference to us.
If we took a million of computers and put them together, the small gravity of those computers would add up to become a huge gravitional field.
So the answer to your question is yes, it would have gravitional pull.(3 votes)
Video transcript
When you look at footage or
photographs of astronauts in space, it doesn't look like
there's any gravity at work here. Everything is not falling
down in one direction. In fact, it's not even
clear what up or down is. Everything just floats around. If I were to push
off of this wall, I would just float
in that direction. So it doesn't look like there's
this overarching influence like gravity that's trying
to pull everything down. But the question is that
these astronauts are still not too far away from
a supermassive body. In fact, the space shuttle
gets up only a couple of hundred miles above
the surface of the Earth. So the space shuttle, if I
were to draw it to scale, would probably be
right about there. And we know that the force of
gravity between two objects is equal to big G, the
gravitational constant, times the mass of the first
object, times the mass of the second object over
the distance between the two objects squared. And if the space
shuttle is right here, only a few hundred miles above
the surface of the Earth, this r isn't that different. It's a little bit
further than if you were at the surface
of the Earth. Remember that r is measured from
wherever you are to the center, from the center of
the Earth, or really the center of the object
to the center of the Earth. The center of the Earth
represents most of the distance here. So if I'm at the
surface of the Earth or if I'm just a few hundred
miles above the surface of the Earth, it's not going
to change r that dramatically, especially in terms
of percentage. So when you look at it this
way, it seems pretty clear that the force of
gravity for someone who is in space only a few
hundred miles above the Earth should not be that different
than the force of gravity for someone who is on
the surface of the Earth. So my question to
you is, what gives? If there should be
gravity in space, how can we see all
of these pictures of people floating
around like this? And the answer is that
there is gravity in space, and that these people
actually are falling. They're just moving fast
enough relative to the Earth that they keep missing it. And let me show you what
I'm talking about there. Let's say I'm sitting
here in Africa, and I were to shoot
something, if maybe I have a really good
sling shot, and I were to sling something super
fast and maybe at a 45 degree angle, it might take
off a little bit and eventually
hit another point. And this would actually already
be a super duper slingshot. I just made it travel a couple
of thousand miles or at least over 1,000 miles. If I make it go a
little bit faster, if I put a little
bit more force on, if I just propelled the
projectile a little bit faster, it might go a
little bit further, but it will eventually
fall back to the Earth. Let's try to propel it a
little bit faster than that. Then it'll still
fall to the Earth. Let's propel it even
faster than that. Well then, it's still eventually
going to fall to the Earth. I think you might see
where this is going. Let's go even faster than that. So if we go even
faster than that, eventually it'll
fall to the Earth. Even faster than
that, so if you were to throw an object
even faster than that, it would then go really far
and then fall to the Earth. I think you see
what's happening. Every time you go
faster and faster, you throw this projectile
faster and faster, it gets further and
further, up to some velocity that you release
this projectile, and whenever it's trying
to fall to the Earth, it's going so fast that it
keeps missing the Earth. So it'll keep going around and
around and around the Earth, and a projectile like that
would essentially be in orbit. So what's happening
is if there was no gravity for that projectile,
if there was no gravity, the projectile would just
go straight away into space. But because there's gravity,
it's constantly pulling it towards the center of
the Earth, or the center of that projectile and
the center of the Earth are being pulled
towards each other, I guess is a better
way to think about it. The force of gravity
is doing that. And so it's curving its path. And if it's going fast enough,
if the projectile or whatever object we're talking about
is going fast enough, it'll just keep going
round and round the Earth. And since there is almost,
pretty much, for most purposes no air if you go high enough,
especially the altitude that the space shuttle
is, no noticeable drag, this thing can
just keep on going for a substantial
amount of time. Although there is just
a little bit of drag, and that's why over time you
do have satellites slow down, because there is just a
little bit of air resistance. So the answer to
this conundrum is that there actually is gravity. It's not a gravity-free
environment. It's just that the astronauts
and the space shuttle and everything else that's
in the space shuttle, it's all falling,
but it's moving fast enough that it
never hits the Earth. It keeps missing the Earth. It keeps going round
and round and round, but it is completely under
the influence of gravity. If they were to just
slow themselves down, if they were to just brake
relative to the Earth, and if they were to just
put their brakes on right over there, they would all
just plummet to the Earth. So there's nothing special
about going 300 or 400 miles up into space, that all of a
sudden gravity disappears. The influence of gravity,
actually on some level, it just keeps going. You can't, it might
become unnoticeably small at some point,
but definitely for only a couple of
hundred miles up in the air, there is definitely
gravity there. It's just they're in orbit,
they're going fast enough. So if they just keep
falling, they're never going to hit the Earth. And if you want to
simulate gravity, and this is actually how
NASA does simulate gravity, is that they will put
people in a plane, and they call it
the vomit rocket because it's known
to make people sick, and they'll make them go
in a projectile motion. So if this is the ground,
in a projectile path or in a parabolic
path I should say, so the plane will
take off, and it will do a path exactly the
same as something in free fall or in a parabolic path. And so anyone who's
sitting in that plane will experience free fall. So if you've ever been
in, if you've ever right when you jump off of a
or if you've ever bungee jumped or skydived
or even the feeling when a roller coaster is
going right over the top, and it's pulling you
down, and your stomach feels a little ill, that
feeling of free fall, that's the exact same feeling
that these astronauts feel because they're in a
constant state of free fall. But that is an
indistinguishable feeling from, if you were just in deep space
and you weren't anywhere close any noticeable mass, that is
an identical feeling that you would feel to having no
gravitational force around you. So hopefully that clarifies
things a little bit. To someone who's just
sitting in the space shuttle, and if they had no
windows, there's no way of them
knowing whether they are close to a massive
object and they're just in free fall around it, they're
in orbit, or whether they're just completely far away
from any massive object, and they really are in
a state of or in a place where there's very
little gravity.