|How can there be gravitational pull in space but
no gravity? |
|Question Date: 2014-10-03|
Well, that depends on what you mean by
"gravitational pull" and "gravity".
"Gravity" and "gravitational pull" usually mean
the same thing - they just refer to the force
between two masses. And both of those
definitely exist in space. I think what you
mean is "weightlessness" - that is, that
when people go into space, they float and have no
So what causes things in space to be
weightless? Well, let's ask a different
question: what causes you to feel like you have
weight? When you're standing (or sitting) on the
surface of the Earth, the force of gravity is
pulling you towards the center of the Earth. So
why aren't you falling down? Well, it's because
the ground you're standing on is also pushing back
on you and holding you up. It's that upward push
from the ground that you actually feel as your
Now, what about when you're falling freely
(say, while skydiving)? Well, with no ground
below you to push up on you, you don't feel like
you have any weight, so you feel like there's no
gravity. There really is gravity, of course -
that's what's making you fall - but you can't feel
it. This is exactly what happens in space -
you're always falling under the pull of gravity,
but because there's no ground below you to hold
you up, you feel weightless.
There is gravity in space, it is just weaker
because of the distance away. Objects with mass
have a gravitational pull, even you and I do. The
Earth has a large pull, so it can exert that on
large objects farther away. But still, its
individual gravity isn't very strong at long
distances. In low earth orbit (LEO), like the
international space station, gravity is still
fairly strong, so why are astronauts weightless?
You would have to be deep in space to escape
Earth's gravity fully. The reason has to do with
being in orbit. Think of being inside a plane in
freefall. You and your environment would be moving
downward so rapidly it would appear that you were
here to see.
The space station is falling in the same way.
But, it is falling at an angle so that it
continues to fall in circles around the Earth, if
that makes sense, like the rock in this picture
It continues to "fall" around the Earth as it
orbits, making the astronauts appear weightless.
Gravity is force between two objects. Gravity,
is defined as
Where m1 and m2 are the mass of the two objects
attracting each other and r is the distance
between the two objects; G is a constant that
depends on the units specified for m1, m2, r, and
F. What most people think of when they hear
the word gravity is weight, which is a force
preventing someone from accelerating towards the
center of a mass.
In space one doesn’t feel the weight of
gravity, because there is no surface, such as the
surface of the earth push against them. It is the
same effect as free fall.
Note: gravity falls off as
1/r2. Try plugging in different values
for r in the equation and see how quickly it can
change the force of gravity.
Actually, there IS gravity in space -- there is
gravity everywhere! Perhaps the question you
would like to ask is why astronauts feel
weightless in space.
Consider for instance that the gravity
experienced at the altitude of the space station
is 90% of the gravity experienced at the Earth's
surface (so a person who weighs 100lb at the
surface of the Earth and climbs a ladder to the
altitude of the space station would weight 90lb).
Trippy, right? So why do astronauts at the space
station feel weightless? The weightless sensation
that astronauts experience in space is a
consequence of the fact that there are no objects
in contact with their bodies to exert a push or
pull ("contact forces"). In other words, the only
force they experience is a "non-contact" force
It is through contact forces that people
perceive weight. When a person stands on the
ground on earth, there is a downward force being
exerted on him or her (the person's mass times the
gravitational constant). The ground in turn exerts
a "normal force" (upward on the person's feet)
that balances the the downward force (if it
didn't, the person would fall through the ground).
To clarify this idea, let's consider putting
this person in an elevator. Now, the person is
falling at the same rate as the Earth is, so the
person has some inertia, which can be stated as
the tendency of an object to continue moving in
the same direction it is already moving. Thus, the
person's inertia would prefer if the person
remains stationary. In order to move the person up
in the elevator, the elevator must exert more
normal force on the person's feet than the person
would feel on ground that isn't accelerating
upward. Thus, the person's perceived weight is
higher. If the elevator is accelerating downward,
the normal force pushing on the person's feet is
less than the person's weight on the ground (hence
accelerating downward), so the person's perceived
weight is less than their perceived weight on the
So how does this relate to why astronauts feel
Astronauts in a space station that is orbiting
Earth are accelerating ("falling") toward Earth at
the same rate that the space station is (since
space is a vacuum, the mass of the object does not
affect its acceleration -- all objects accelerate
at the same rate). Because of this, astronauts do
not experience the contact forces necessary for
them to perceive weight. I hope that helps!
There is gravity in space.
If everything around you is subjected to the
exact same gravity as everything else, then it
will create the illusion that there is no
gravity. So, standing on Earth, the ground
underneath you is supported by the rocks
underneath it, thus resisting the force of
gravity, so you stand on the surface of the Earth,
recognizing fully that gravity exists. In orbit,
or even on amusement park rides where you are
(briefly) falling, you and everything else
orbiting/falling with you will seem weightless,
because you are all orbiting/falling in the same
direction at the same rate.
As I said, this is an illusion. The
International Space Station experiences *most* of
the same gravity that we do down at the surface,
but since it is all orbiting/falling together, an
astronaut on the space station will not notice
that there is any gravity because there set of the
space station experiences the same acceleration.
So there is gravity in space, it is just very
weak. Gravitational pull between 2 objects is
proportional to how large the objects are. It is
also proportional to how far the two objects are
apart from each other. For a very large object
like the Earth and a very small object like a ball
that are close together, the gravity can be pretty
strong. However, for a relatively small object
such as a space craft that isn’t really close to
any large object, the gravity isn’t that strong.
As soon as the space craft gets near a planet or a
moon though, the distance becomes small enough
that gravity becomes significant and it is pulled
towards the large body. Technically speaking,
there is gravity in space, it just exerts such a
small force that it doesn’t move anything and
therefore seems like it doesn’t exist.
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