# B Weightlessness in space

1. Apr 5, 2017

### Vengo

Another Doubt
Guys, I was watching this video on youtube. it is about weightlessness in space. it states that the astronauts inside a space ship 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 space ship are not pulled by earth's graviy???

2. Apr 5, 2017

### Staff: Mentor

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

3. Apr 5, 2017

### 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.

4. Apr 5, 2017

### A.T.

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

5. Apr 5, 2017

### Staff: Mentor

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 space ship 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.

6. Apr 5, 2017

### ZapperZ

Staff Emeritus
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.

7. Apr 5, 2017

### Vengo

then why they didnt fall

8. Apr 5, 2017

### Vengo

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

9. Apr 5, 2017

### Staff: Mentor

The do fall - at the same rate whether inside outside. That's what an orbit is! (falling forever and never hitting earth).
Inside the ship or out, the zero *apparent* gravity is due to being in orbit.

10. Apr 5, 2017

### Vengo

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 space ship ??

11. Apr 5, 2017

### ZapperZ

Staff Emeritus
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.

12. Apr 5, 2017

### Staff: Mentor

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.

13. Apr 5, 2017

### Vengo

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

14. Apr 5, 2017

### Vengo

Then the motion is relative,

15. Apr 5, 2017

### Staff: Mentor

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

16. Apr 5, 2017

### Vengo

I am really sorry I cant get you

17. Apr 5, 2017

### Staff: Mentor

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

18. Apr 5, 2017

### Staff: Mentor

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

19. Apr 5, 2017

### 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

20. Apr 5, 2017

### A.T.

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

No, the centripetal acceleration is due to gravity.

21. Apr 5, 2017

### Vengo

Thank you

22. Apr 7, 2017

### mic*

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.

23. Apr 7, 2017

### 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.

24. Apr 7, 2017

### 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...

25. Apr 7, 2017

### A.T.

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