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In the scene where Dr. Ryan Stone's leg is entangled in Soyuz's parachute cords and she is tethered with Matt Kowalski through a strap, there is tension in the strap, suggesting act of some external force. Where is this force coming form that is causing tension in the strap and pushing both Dr. Ryan and Matt outwards?

I'm guessing that the tension might be coming from centrifugal force, but immediate events before this scene, as well as other parts of ISS in same scene do not suggest presence of such force (or presence of any kind of rotation).

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While I for myself am not sure if there was any tension (especially a continuous tension pulling him away even after his motion has been stopped by Dr. Stone), this interview with a real astronaut suggests there wasn't, especially this section:

We see quite a few worst-case scenarios in the film. George Clooney ends up untethering himself from his crewmate and literally floating away into space. Is dealing with that scenario part of your training? Or does that not even come up?

Actually, I really couldn't understand the tension there. Sandra has her leg caught in some parachute rigging, and she's holding on to George by his hand. I think all he would have needed to have done would be to crawl up on her, and basically rope-climb up toward the space station. Or just lightly pull and he could have flown himself up to the space station. There would be no continuing motive force to continue to pull him away. So that physically didn't work for me.

So when George he let go of her hand, if it was in real space, he wouldn't have been pulled away.

He would have just floated there.

So according to the above interview the answer to your question would be "nothing".

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And again the default answer to all those Gravity questions: "it wasn't as scientifically accurate as it might have suggested". (just don't let that ruin this amazing movie for you, it wasn't a documentary anyway). –  Napoleon Wilson Nov 12 '13 at 9:14
    
Yet nobody knows if the astronaut from this interview just didn't understand that scene entirely, but we just have to assume he knows what he's talking about. –  Napoleon Wilson Nov 12 '13 at 9:24
    
Well obvious answer but showed a good research work, +1. –  Ankit Sharma Nov 12 '13 at 11:02
    
@AnkitSharma "showed a good research work" - Well, that's just the same link I always throw at this kind of Gravity-questions. –  Napoleon Wilson Nov 12 '13 at 12:47
    
Didn't realized. Now i know where to look for answering next gravity question ;P –  Ankit Sharma Nov 12 '13 at 13:06
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During that scene, Stone's leg gets wrapped up in some parachute cabling. She then starts slowly rotating around whatever the cabling is stuck on. Then she grabs the strap attached to Kowalski. She's still rotating. In order to hold onto him, she's providing a centripetal force. After releasing him, she finally rotates into another part of the station, and works her way to the airlock.

I look forward to being able to provide some evidence of this once the film is released on blu-ray.


Basic physics:

image of 2 astronauts in scene

We have 4 things of import in this image.

  1. Stone's ongoing motion, which are the lines behind her in my crude drawing.
  2. A, the force that Stone is exerting on Kowalski.
  3. B, the force Kowalski is exerting on Stone. This is equal and opposite to A.Newton's 3rd Law If it wasn't equal and opposite, then Kowalski would be moving toward or away from Stone at this moment.
  4. C, the centripedal force exerted on Stone by the parachute cabling wrapped around her foot. This force is going to be equal to A + the amount of force needed to keep Stone alone moving in the arc.

C is larger here due to it needing to pull the mass of both Kowalski and Stone. Once Kowalski is released, then C just needs to pull Stone, which drastically reduces C's magnitude.

In the scene, while C is large, Stone's leg is slipping out of the parachute cabling. Once C is reduced, this stops, and she soon swings into the hull of the ISS.

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Makes sense, though her holding him should deccelerate (is that a word?) his movement until he stops being dragged away, shouldn't it (but I'm far from an expert in orbital mechanics either)? –  Napoleon Wilson Nov 12 '13 at 18:01
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They're both rotating still. Until something stops that movement, she needs to maintain that centripetal force. –  Keen Nov 12 '13 at 18:05
    
I still don't understand why Clooney had to let go. It seemed he could just crawl back or pull himself in –  Huangism Dec 11 '13 at 19:19
    
@Huangism There, I added a drawing and explanation. Further questions can be directed to me or Physics.SE. –  Keen Dec 11 '13 at 19:38
    
@Keen ok I understand the force part, I am going to watch that part more carefully next time. I thought the slip of her feet was because of something else –  Huangism Dec 11 '13 at 20:02
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My guess is that even though the parachute lines had stopped slipping on Stone's leg, the parachute itself was still slipping through the wreckage, and Stone and Kowalski still had momentum directed away from the wreckage. The friction of the parachute against the wreckage was slowing their momentum and causing the tension. Kowalski was facing the parachute. Maybe he estimated that the friction was enough to gently stop one person but not two, before the parachute pulled all the way through the wreckage, or before the line connected to the escape vehicle pulled taut, at which point the lines might be dislodged from Stone's leg. And rather than trying to explain all that, Kowalski would probably try to be more succinct, which he was.

The film was not really successful in getting this idea across, but I'm sure we can agree it is a difficult scenario to depict. I personally am able to suspend my disbelief that far.

It doesn't help that the director is trying to tell the story using long, uninterrupted takes. He doesn't have the option of the usual tension-building technique of cutting to a closeup of the parachute slipping free of one outcropping, then another, etc.

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"Basic Physics" in zero-g works different: once Stone and Kowalski have their momentum stopped by the strap connected to ISS, the 1st law requires Kowalski to float in place once disconnected, which he did not need to do, as a simple tug on the strap would have propelled him toward the ISS. THERE WAS NO FORCE APPLIED WHICH WOULD HAVE ACCELERATED HIM AWAY, AS SHOWN!

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