Why are astronauts outside a spaceship not pulled by Earth's gravity?

[Vengo]

Another Doubt
Guys, I was watching this video on youtube. it is about weightlessness in space. it states that the astronauts inside a spaceship fly in space not due to zero gravity but due to centripetal force with which the satellite is revolving the earth. My question is "then why the astronauts outside the spaceship are not pulled by Earth's graviy?

this is the link

 

[Doc Al]

My question is "then why the astronauts outside the spaceship are not pulled by Earth's graviy?

Astronauts, whether inside or outside the ship, are pulled by gravity just like the ship itself is.

 

[Grinkle]

The astronaut is also in the same orbit - the person is not needing to be 'pushed' by the space station. They were put into orbit by the space shuttle, and once in a stable orbit, they will stay there. If they are in an identical orbit to the space station, they can move around in it etc and look weightless inside the space station as they do so. But once placed into orbit by the space shuttle, they would stay there, with or without a space station nearby that they might crawl inside of.

 

[A.T.]

My question is "then why the astronauts outside the spaceship are not pulled by Earth's graviy?

They are. Why should it matter whether they are outside or inside the ship?

 

[Nugatory]

It's never a good idea to try to learn physics from youtube videos, but this one is not bad. The problem is that you've misunderstood it - it does not say that "the astronauts inside a spaceship fly ... due to centripetal force with which the satellite is revolving the earth." They float inside the station because they and the station are both in free fall, moving freely under the influence of gravity.

 

[ZapperZ]

Another Doubt
Guys, I was watching this video on youtube. it is about weightlessness in space. it states that the astronauts inside a spaceship fly in space not due to zero gravity but due to centripetal force with which the satellite is revolving the earth. My question is "then why the astronauts outside the spaceship are not pulled by Earth's graviy?

Now think about this a bit. Let's say you have a man in an elevator on Earth in a free fall. There's another man next to him, outside of the elevator, who also jumped at about the same time as when the elevator+man starts to free-fall.

Ignoring air resistance, do you think they will fall at different rate? Doesn't the man in the elevator experiences weightlessness the same way as the man outside the elevator?

If the astronaut is orbiting the earth, he/she and all the contents of his body are also orbiting the earth, whether he/she is inside, outside, on top, on the side... of any vessel. This means that the centripetal force in the form of gravitational force is acting on all of them, and they are all having the same centripetal acceleration. Thus, the weightlessness.

Zz.

 

[Vengo]

Astronauts, whether inside or outside the ship, are pulled by gravity just like the ship itself is.

then why they didnt fall

 

[Vengo]

It's never a good idea to try to learn physics from youtube videos, but this one is not bad. The problem is that you've misunderstood it - it does not say that "the astronauts inside a spaceship fly ... due to centripetal force with which the satellite is revolving the earth." They float inside the station because they and the station are both in free fall, moving freely under the influence of gravity.

Ok then, How astronauts float in outer space? is it due to zero gravity?

 

[russ_watters]

then why they didnt fall

The do fall - at the same rate whether inside outside. That's what an orbit is! (falling forever and never hitting earth).

Ok then, How astronauts float in outer space? is it due to zero gravity?

Inside the ship or out, the zero *apparent* gravity is due to being in orbit.

 

[Vengo]

The do fall - at the same rate whether inside outside. That's what an orbit is! (falling forever and never hitting earth.

For a Space ship, it revolve around the orbit due to the orbital velocity. But a man outside space. Is he moving at the same orbital velocity as the spaceship ??

 

[ZapperZ]

For a Space ship, it revolve around the orbit due to the orbital velocity. But a man outside space. Is he moving at the same orbital velocity as the spaceship ??

Is here next to the ship all the time? Astronauts that do space walk in the ISS are in a relatively stationary frame as the ISS. If the ISS is orbiting the earth, doesn't this automatically imply that so does the astronaut?

Zz.

 

[russ_watters]

For a Space ship, it revolve around the orbit due to the orbital velocity. But a man outside space. Is he moving at the same orbital velocity as the spaceship ??

Well sure; he got outside by climbing out through the airlock. He got into orbit with the ship, in the same way it did; by sitting atop a giant rocket. He's not wearing a giant rocket on his back when climbing out the airlock, so he has no way to greatly change his orbital velocity.

 

[Vengo]

View attachment 123630

This means that the centripetal force in the form of gravitational force is acting on all of them, and they are all having the same centripetal acceleration. Thus, the weightlessness.

Zz.

I can understand you but how does the centripetal acceleration acts for person outside the space ship. In spaceship it is due to velocity with which it rotates.
So will the person outside the spaceship will also have the same velocity?

 

[Vengo]

Well sure; he got outside by climbing out through the airlock. He got into orbit with the ship, in the same way it did; by sitting atop a giant rocket. He's not wearing a giant rocket on his back when climbing out the airlock, so he has no way to greatly change his orbital velocity.

Then the motion is relative,

 

[Doc Al]

So will the person outside the spaceship will also have the same velocity?

They'd better, don't you think? If they want to go home!

 

[Vengo]

They'd better, don't you think? If they want to go home!

I am really sorry I can't get you

 

[russ_watters]

Then the motion is relative,

Sure, motion is always relative. The motion that matters here is the orbital speed, which doesn't change when the astronaut exits the airlock.

 

[russ_watters]

I am really sorry I can't get you

If the astronaut has a different velocity than the spaceship they will move apart, and the astronaut won't be able to go home.

 

[Vengo]

Thank you all for clearing my doubt
I find this forum very helpful and I am going to use it to clear all my doubts

 

[A.T.]

I can understand you but how does the centripetal acceleration acts for person outside the space ship.

Just like inside, the centripetal force is provided by gravity.

In spaceship it is due to velocity with which it rotates.

No, the centripetal acceleration is due to gravity.

 

[Vengo]

No, the centripetal acceleration is due to gravity.

Thank you

 

[mic*]

Just like inside, the centripetal force is provided by gravity. No, the centripetal acceleration is due to gravity.

I don't think this is a clear statement. The centripetal acceleration is a consequence of rotational motion.

The rotational motion of an orbit arises from a balance between the centripetal force (gravitational attraction to the Earth in this case) and the tangential velocity. Newton proposed such a balance in his original thought experiment about (planetary) orbital motion.

 

[mic*]

Vengo, if the astronaut outside the ship did not have the critical tangegtial velocity (which will be almost identical to that of a space ship) he will dramatically fall to Earth - like they animate in the video.

The question is, how would he get there - in space, without a ship?

If it is by climbing outside a real one (this is possible) then he will continue to orbit with the ship.

If he got magically teleported 500km straight up from the surface, he will fall dramatically back down. This imaginary idea is what the video has supposed.

 

[mic*]

As an aside, here is what happens when you don't achieve orbital tangential velocity...
Might be the closest you will get to being in space without a ship...

 

[A.T.]

The rotational motion of an orbit arises from a balance between the centripetal force (gravitational attraction to the Earth in this case) and the tangential velocity.

I don't think talking about a "balance" between a force and a velocity makes much sense.

 

[mic*]

Fair point. Poor semantic choice when balanced forces are frequently spoken of.

For an object in a stable orbit tangential velocity squared is equal to the centripetal force times the radius.

If this equation does not hold, orbit is not stable.

 

[jbriggs444]

Fair point. Poor semantic choice when balanced forces are frequently spoken of.

For an object in a stable orbit tangential velocity squared is equal to the centripetal force times the radius.

If this equation does not hold, orbit is not stable.

If that equation does not hold, the orbit is not circular. It can still be perfectly stable -- and elliptical.

 

[mic*]

^ Fair point also.

Kepler's laws give the generalised equations.

Do you think it was necessary to introduce that complexity, or perhaps you feel it was unreasonable to omit to mention again the context of objects in low Earth orbit?

 

[jbriggs444]

^ Fair point also.

You can use the quote feature to reference the post (or relevant extract thereof) to which you are responding -- as I have done here.

At least three variants of this functionality exist (at least on a browser platform -- which is the interface I use).

1. You can click "Reply". In the editing window, the text of the post to which you are responding will appear, complete with [QUOTE] and [/QUOTE] tags. You can edit the quoted text within those tags down to just the passage to which you wish to respond.

2. You can mouse over a passage to which you want to reply, highlighting it and then click "Quote" to add that passage to your quote buffer. In the message editing window you will then have an "Insert Quotes" button available to insert that quote, complete with tags.

3. You can go to Info => Help/How To => BBcodes and find the [QUOTE] functionality described there. Basically you manually put a [QUOTE] tag in front of the quoted section and a [/QUOTE] tag after it.

In any case, the "Preview" button will let you see whether you have successfully rendered a quote or whether you have stuffed things up.

Back to the matter at hand. You had asked:

Do you think it was necessary to introduce that complexity, or perhaps you feel it was unreasonable to omit to mention again the context of objects in low Earth orbit?

As others have pointed out, Earth orbit, low, high, elliptical, circular, hyperbolic or otherwise is irrelevant. What matters is that the force of gravity on a person and on the spacecraft that is nearby are very nearly equal. The person and craft will be moving and accelerating at very nearly identical rates. If the person uses the spacecraft as his reference to know whether he is moving or accelerating then he will see that he is not moving or accelerating significantly. Whatever force might exist from gravity appears to have very nearly no effect at all.

I responded because you had made a confusing and incorrect statement equating "stable" with "circular".

 

[mic*]

Thankyou for pointing out the quote feature. Should I have used it now, or will chronological progression of posts suffice?

Without tangential velocity there is no centripetal acceleration, just straight line acceleration due to gravitational attraction. The vector for this acceleration is not even really a variable in this simplified circular low Earth orbit context, if such a simplification is permitted.

Since 2:30-2:45 in the OP video answers the OP question directly, discussing tangential velocity as I have, I thought it was reasonable to draw the conversation back to this point and then address the possible confusion that can arise from the counterintuitive scenario where a man can travel 25,000km/hr without being blanketed in a metallic shell, or ripped to pieces.

 

[mic*]

I responded because you had made a confusing and incorrect statement equating "stable" with "circular".

I did not equate stable with circular. I provided the formula for a stable circular orbit without explicitly stating that it was a simplification only applicable to circular orbits.

 

[jbriggs444]

I did not equate stable with circular. I provided the formula for a stable circular orbit without explicitly stating that it was a simplification only applicable to circular orbits.

Fair enough. With that understanding, I can withdraw my quibble.