Question about gravity

wylo

Sorry if this sounds like a dumb question , but its something ive often wondered about gravity. Why is it there seems to be a particular point where the strong gravitational pull is lost as it leaves Earth?
What I mean is, relatively speaking , astronauts are very close to Earth yet seem to experience NO gravity. I would have imagined it would be a gradual process? Almost like as if they should be slightly falling towards Earth, just not as much as when on Earths surface.
I would have imagined that the side of shuttle or whatever that is facing Earth would be the side the astronauts would be more pulled to if you get me, as opposed to them floating aimlessly.
Now I do understand that you would naturally orbit earth and if you werent going at such a speed you would eventually 'fall' into Earth due to gravity but i dont get how the pull is so weak at such a near distance, the change seems very very sudden, like going from experiencing the full effects of gravity,i.e. falling, straight to floating. I thought it would make more sense that would go like this (leaving Earth), falling -> falling -> slightly falling but at a slower speed -> practically no falling just a bit of a pull -> no pull whatsoever (like beyond the moon kind of distance)

Astronauts in orbit don't feel the effects gravity because they're constantly in free-fall. It's not that the Earth's gravity isn't having an affect on them.

If you were in an elevator in a very, very tall building, and the elevator suddenly went into free fall, it would appear to you that you're floating relative to the elevator, that essentially you're not affected by gravity; that you're flying. If there were items in the elevator with you, they'd float as well. This is essentially the same with an astronaut in orbit around the Earth. The Shuttle or satelitte or whatever spacecraft it is that's in orbit is constantly free falling towards Earth, but it's also moving "forwards" at a great speed too, so by the time it has gotten to the place it was falling to, the Earth hasn't gotten any closer, because the Earth is curved. The astronaut appears to be floating relative to the Shuttle for the same reason as you'd appear to float relative to the elevator. It's a difficult concept to explain. This video might help you.

On a side note, the force of gravity at the surface of the Earth is approx. (9.8)x(your mass), at the height of an orbit, the force is roughly (8.8)x(your mass). So, as you can see the difference in gravitational force at 350km (the height of a Shuttle orbit) is roughly 10%.

thecommander

From what I know astronauts are in micro gravity, they're still being pulled in just at a very slow pace.

http://en.wikipedia.org/wiki/Weightlessness#Microgravity

wylo

-JammyDodger- wrote: »

Astronauts in orbit don't feel the effects gravity because they're constantly in free-fall. It's not that the Earth's gravity isn't having an affect on them.

If you were in an elevator in a very, very tall building, and the elevator suddenly went into free fall, it would appear to you that you're floating relative to the elevator, that essentially you're not affected by gravity; that you're flying. If there were items in the elevator with you, they'd float as well. This is essentially the same with an astronaut in orbit around the Earth. The Shuttle or satelitte or whatever spacecraft it is that's in orbit is constantly free falling towards Earth, but it's also moving "forwards" at a great speed too, so by the time it has gotten to the place it was falling to, the Earth hasn't gotten any closer, because the Earth is curved. The astronaut appears to be floating relative to the Shuttle for the same reason as you'd appear to float relative to the elevator. It's a difficult concept to explain. This video might help you.

On a side note, the force of gravity at the surface of the Earth is approx. (9.8)x(your mass), at the height of an orbit, the force is roughly (8.8)x(your mass). So, as you can see the difference in gravitational force at 350km (the height of a Shuttle orbit) is roughly 10%.

Cheers , that explains it , so basically it doesnt really matter whether the spacecraft is in space or not its the 'freefall' towards Earth thats keeping them floating?

In theory , if you were to get a giant platform 350km high, could an astronaut stand on it like he could if it was just 1km high even though hes in space? (bar the 10% loss in force)

wylo

wylo wrote: »

Cheers , that explains it , so basically it doesnt really matter whether the spacecraft is in space or not its the 'freefall' towards Earth thats keeping them floating?

Exactly, well, an important factor is the lack of air resistance. There's negligible air resistance at 350km, so that's how the speed required to stay in orbit can be achieved (28,000 km/hr).

In theory , if you were to get a giant platform 350km high, could an astronaut stand on it like he could if it was just 1km high even though hes in space? (bar the 10% loss in force)

If there was no air resistance, I don't see why not. I'm open to correction on that, though.

Beeker

wylo wrote: »

Cheers , that explains it , so basically it doesnt really matter whether the spacecraft is in space or not its the 'freefall' towards Earth thats keeping them floating?

In theory , if you were to get a giant platform 350km high, could an astronaut stand on it like he could if it was just 1km high even though hes in space? (bar the 10% loss in force)

Correct! As far as my knowledge tells me:)

FISMA

There's a lot to discuss here, I will try to be brief.

Gravity is a property of matter. Every piece of mass in the universe pulls on every other piece of mass in the Universe with an equal and opposite force. Gravity acts at the speed of light and is an infinite ranged Force.

There is no point where the Earth's Gravitational Force is zero. You could theoretically say that the field is zero at infinity though. There are places where gravitational Forces balance out and give no net Force. However, Gravity acts continuously throughout spacetime.

Gravity is a force that acts over all distances: its range is infinite. No matter where a body is located, there is gravity is present attempting to accelerate the body.

Astronauts have a significant amount of gravity acting on them during low earth orbit. Gravity obeys an inverse square law. If you go two earth radii from the center of the Earth, the force of gravity decrease by a factor of four. Go 3 Earth Radii, the force decreases by a factor of 9.

As a rule of thumb, for every 30 km above sea level, the acceleration of gravity decreases by 1%. Thus, in low earth orbit, 300 km, gravity has decreased 10%. An astronaut that weighs 150 lbs at sea level (667 Newtons) weighs 135 lbs (660 Newtons) in orbit.

A term that has come into use lately is micro-gravity. Micro is the metric prefix meaning one millionth. Such a term helps in that it reiterates there is gravity in space. However, it hurts by implying the acceleration due to gravity is insignificant which it is not.

Gravity does not abruptly fall off, but decays exponentially.

Astronauts most definitely do feel the effects of gravity. In fact, it is the only reason they are going in a circle. Objects will not naturally want to travel a circular path (Newton's 1st). Objects must be forced to go in a circle. In order to go in a circle you must have an inward acting Force. We call this a centripetal Force. Centri - center, petal - seeking.

The centripetal force is not a new force, there's only four (gravity, E&M, Nuclear Strong, Nuclear Weak). Rather, the centripetal force is a description of a Force.

When you go around a circle in a car - friction IS the centripetal Force. When you twirl a yo-yo on a string in a circle - tension IS the centripetal Force. When astronauts go around the Earth in a circle - Gravity IS the centripetal Force.

When you are in outer space in orbit you are falling back to the Earth. The problem is that you are going so fast that it's going to take a while. There is a bit of friction due to particles in outer space. Over time these particles cause noticeable drag. Thus, the orbits of satellites are decaying and they eventually crash back to Earth.

Technically, you are correct that the side of the shuttle closest to the Earth is in a greater gravitational potential, however, the difference is negligible.

Gravity is greatest at sea level and decreases as you go up or down. Technically, in a symmetric uniform spherical planet, the net effect of gravity is zero at the center of the Earth.

Your head is in a lower gravitational potential than your feet - technically. However, I doubt there exists any measuring device accurate enough to demonstrate the difference.

The differences do not start to show up until you go large distances - like one, two, or three Earth radii from the center of the Earth.

What is going on is that the astronauts "feel" weightless. If they get on a scale on the space shuttle, the scale reads zero. Thus, they claim they are weightless. However, if you go with the definition of Weight = mass*acceleration due to gravity (not really g), then you will find that astronauts are 90% of their sea level weight in orbit.

The shuttle is falling, the scale is falling, and the astronauts are falling. If you are falling and the scale is falling at the same rate, then the scale cannot work as it is designed to be on solid ground.

This is where the term "apparent" weightlessness comes in. You are still most definitely weighted. However, you apparently fell and experience otherwise.

Also, you are most definitely not floating but falling.

The movie Apollo 13 was filmed aboard an airplane that repeatedly flew along a sine wave: flying up and then going into free fall. The flight path is what provided the realistic appearance of being in space. Movie patrons thought the effects were very good; describing them as being "just like" in outer space.

The "effect" of weightlessness was not "like" being in outer space, it is the same exact thing. Astronauts in outer space are in free fall, they are falling back to the Earth. The astronauts take a long time to fall back to Earth due to their high tangential speed.

On the airplane during filming, the plane simply flew up to a certain altitude and then went into free fall. The astronauts were never weightless: their mass was constant and gravity was close to that at sea level. However, if one stood on a scale the reading would have been zero. A scale fails to read since the scale is falling at the same rate as the person.

Note of caution - watch out for those Potential Energies. When you are dealing with First year Physics roller coaster problems Gravitation Potential Energy Increases with height GPE = mgh. However, that assumes that you are so close to the surface of the Earth that g = 9.80m/s^2, or whatever local value g is.

However, technically GPE "increases" (becomes less negative) to zero at an infinite distance from the Earth.

One of the major problems here is that our minds cannot handle accelerated reference frames. From your point of view, you are floating, from another, you are falling. The reality is that you are falling.

So much for being brief.:)