Gravity - The film - How did.. "MASSIVE SPOILER!"

[Sokar408]

.. Matt get riped away? Ryan had him and, as far as we saw, stopped his momentum, so what was mysteriously pulling him off?

Great movie even with the small mistakes here and there, but something this plot important I'd like to have cleared up

[Karriz]

I think they were rotating a bit so there was some centrifugal force. At least that's how I explained it to myself.

[Ralathon]

.. Matt get riped away? Ryan had him and, as far as we saw, stopped his momentum, so what was mysteriously pulling him off?

Great movie even with the small mistakes here and there, but something this plot important I'd like to have cleared up

IIRC the entire thing was spinning. They where on the end of a large spinning tether so they would've felt quite a bit of force.

[Codz]

Actually, any centrifugal force would have been so minor at that point that him letting go was pointless. All she needed to do was give him the slightest tug and momentum would have done the rest. The entire scene is simply there to set up Ryan for her "lone survival against all odds".

[Albert VDS]

To me it didn't look like the station debris was spinning them in a way so that they would be flung away from the station.

In the part before Ryan grabs Matt's tether you could see that she was right above the axis of the spin.

In other words; they would have been spinning from back to front and not from top to bottom.

When hes about to unhook you can see a wide shot of the station, them and the earth and they are all stationary.

It was the biggest what the heck moment for me. She would only had to give a small pull to get him closer.

[K^2]

A tether cancels only radial motion. Any angular momentum they'd have flying by the station, they'd still have. And it would take a very small amount of angular motion to generate enough centrifugal force for there to be a problem. They didn't show it all that great, but that's what would have happened, yes.

[Codz]

Another significant plot point happens when Clooney and Bullock reach ISS. Still attached by a tether, they have a hard time finding a grip on the station to stop themselves. Eventually, Bullock’s leg gets tangled in the parachute shroud line from the Soyuz escape capsule. Its hold is tenuous, and she struggles to hold on to Clooney as he is pulled away from her. As her leg starts to slip, Clooney unclips his tether and falls away to his doom, saving her in the process. Except, well, not so much. The thing is, they very clearly show that when Bullock’s leg got tangled up in the shroud line, both her and Clooney’s velocity relative to the space station was zero. They had stopped.

On Earth, if one person is hanging by a rope and holding on to a second person, yeah, gravity is pulling them both down, the upper person bearing the weight of the lower one. If the upper person lets go, the other falls away. But in orbit, they’re in free-fall. Gravity wasn’t pulling Clooney away from Bullock; there were essentially no forces on him at all, so he had no weight for Bullock to bear! All she had to do was give the tether a gentle tug and Clooney would’ve been safely pulled toward her. Literally an ounce of force applied for a few seconds would’ve been enough. They could’ve both then used the shroud lines to pull themselves to the station. This is a case where our “common sense†doesn’t work, because we live immersed in gravity, pulled toward the center of the Earth, supported by the ground. In space, things are different. During that scene, knowing what I know, all I could do was scream in my head “CLOONEY DOESN’T HAVE TO DIE!†but it was to no avail. My publicly admitted man-crush on Clooney plus my not-so-inner physics nerd made that scene hard to watch.

(Quote by Phil Plait, PhD)

http://www.slate.com/blogs/bad_astronomy/2013/10/04/ba_movie_review_gravity.html

[Nibb31]

A tether cancels only radial motion. Any angular momentum they'd have flying by the station, they'd still have. And it would take a very small amount of angular motion to generate enough centrifugal force for there to be a problem. They didn't show it all that great, but that's what would have happened, yes.

I don't buy that explanation.

If they had any angular momentum, it was apparent that they were stationary relative to the ISS. Let me demonstrate my poor drawing skills:

In this magnifiscent work of art, for them to appear stationary relative to the ISS as they were on screen, the station must have been spinning at a speed v while their own speed must have had a speed v' that corresponds to the exact same rotation period as the station for the length of the tether. If they were going only slightly faster or slightly slower, then the tether would have wrapped around the station and pulled them in. Basically, they had to be "geostationary" above the attachment point of the tether. What are the odds of those two speeds v and v' matching exactly out of pure coincidence?

But ok, this is Hollywood. Let's imagine that their speed did match the exact rotation of the station.

In that case, all they needed to do is to pull on the tether or to climb up it, as shown by the green arrow in my crappy diagram. They would have preserved the same absolute angular momentum v' but because the radius of the tether was different, their relative speed to the anchor of the tether would be much higher. Their speed v' would no longer be the magical "geostationary" speed that it was at the end of the tether and would have wrapped around the station and pulled them towards it in a spiral.

The other point that doesn't make sense in this scene, is that as soon as Clooney detaches himself, Bullock is no longer pulled away from the station. She should have had the exact same angular momentum v' as before, yet she has no problem climbing back up on the tether to get back to the station. She could have done exactly the same thing with Mr Nespresso still attached.

Edited December 4, 2013 by Nibb31

[colmo]

I put it down to a new universal force called "narrativity", which overides other forces when the story demands it.

[lajoswinkler]

Plait is wrong. He obviously watched the movie once and didn't think much about this.

Their momentum was not cancelled and it can be seen in one scene that they're still rotating around the station, although slowly. It is impossible for them to come to a complete halt. It just won't work.

The rope was very long and Kowalski's mass is significant (probably around 120 kg) so the force involved was around 10 newtons, which is significant if you're held by rope clinging to your spacesuit.

This issue has been discussed ad nauseam in this thread.

http://forum.kerbalspaceprogram.com/threads/28361-Gravity-%28Movie

Basically, it's one of the most scientifically accurate scenes in the history of SF filmmaking. Even if you can't see them rotating, they should rotate and the force should exist, but as I've said earlier, it can be seen in one scene.

[Codz]

It's one thing to say it was passable, but to say that it was one of the most scientifically accurate scenes in the history of filmmaking? That's just insulting.

Another huge error was – there was absolutely no reason for Clooney to sacrifice himself!!! Once Sandra caught him, he would be just floating there. A small tug on his tether would send him back to the space station. And as my wife put it, when you have a hold of George Clooney, only an idiot would let him go.

Of course, at the speed he was tumbling by her, there is no way she would have caught him in the first place. He weighs a few hundred pounds and his suit weighs a few hundred more. Try lying on a sheet of ice while a 500 pound weight goes flying by and then try to grab a tether attached to that weight while wearing a pair of gloves from a medieval suit of armor. Not so easy.

(Quote by Astronaut Garrett Reisman)

http://www.forbes.com/sites/quora/2013/10/17/what-does-a-real-astronaut-think-of-gravity/

[Drunkrobot]

I'm willing to back colmo's theory of general narrativity. Yes, it was a very scientifically accurate film, yes it was the best film I've ever seen, and yes, I will be getting the DVD of it for Christmas because it's "film so good you watch it at the cinema and then buy it as a DVD" good, but even this film has to bend physics a little. If Kowalski (side note: Kowalski Kerman=necessary game name. Get it done, Squad!) didn't die, then the plot would've lost quite a bit of the drama (reaching the Chinese space station via landing rockets would've been something he would instantly think of).

One of the reasons, in my opinion, that diamond hard sci-fi films like Gravity and Europa Report don't happen as often is because they enter a sort of "uncanny valley". Star Wars and Star Trek are closer to fantasy (operating under author-created rules i.e. the force, warp drive) than science fiction (works under the laws of nature as we understand them), so we don't mind if they break the laws of physics. But with Gravity, billing itself as realistic, it risks being attacked for making the odd mistake.

I love science, and seeing more stuff like Gravity on the big screen would be brilliant, but I can forgive the makers of the film for bending the rules a little for the sake of plot. I go to school for the perfect stuff!

Edited December 4, 2013 by Drunkrobot

[Codz]

I agree that Gravity was, despite some flaws, a good movie. In my opinion, one of Gravity's greatest strengths was how accurately they modeled the appearance of the equipment and spacecraft.

[lajoswinkler]

It's one thing to say it was passable, but to say that it was one of the most scientifically accurate scenes in the history of filmmaking? That's just insulting.

(Quote by Astronaut Garrett Reisman)

http://www.forbes.com/sites/quora/2013/10/17/what-does-a-real-astronaut-think-of-gravity/

Well he's wrong, too. He assumes they've cancelled each other's momentum, which is not true.

The gravity of the situation (see what I did there?) depends on the strength of the rope clinging to Stone's foot. If they're rotating (they are) and the rope is slipping (it is), a "small tug" could cause the rope to slip, sending Stone and Kowalski on a tangential trajectory away from the station, and that possibility has been used in the movie. Kowalski sees that Stone is about to slip and he decides to let himself go (after giving a speech, but hey, it's a movie).

The great thing I love about science is that your title and credibility fail as arguments if even a kid comes forward with a reasonable explanation.

[Codz]

The great thing about science is that you have to provide proof. "Because I said so" isn't good enough. Care to provide evidence of the severe spinning, beyond a very biased opinion?

[K^2]

It doesn't take "severe" spinning. It takes very little, in fact.

Length of the rope is about 15m. (Estimated from picture on previous page.) They weren't moving too fast before getting caught in the ropes, but fast enough to make it difficult to grab things. Lets go with a low-end estimate of 1m/s. Now, how close did they pass to the point where ropes are attached? Lets be generous and say no more than 1m. The specific angular momentum, then, is 1m²/s. Like I said earlier, ropes would not kill angular momentum. In that case, at 15m from origin, they would retain about 1/15th or about 0.06(6)m/s of transverse velocity. That's not much. In fact, only about 0.05 RPM. This is just a little more than two and a half times faster than a minute hand on the clock.

Lets do forces, shall we? Clooney's character is wearing an EMU. That's about 125kg. Lets go on the skinny side for the astronaut for a nice, round total of 200kg. At the above transverse velocity, this generates 0.06N of force. This is about a weight of a marble here on Earth. Really not a lot. But given that there are absolutely not other forces save for friction in the practically free-floating ropes? It is going to be enough to produce something very similar to what we see in the movie. And keep in mind, this is the absolutely lowest possible estimate.

[LLlAMnYP]

The weight of a marble? Really? Was that tether coated in Teflon and grease, that she struggled to hold that weight?

I'd sooner go with the opinion, that the makers of the movie made an outstanding job of this problem of centrifugal force pulling the astronauts apart being as unconvincing as possible in that scene. I'd like to see that scene again, but I struggled to find it on youtube (bummer...), however in the theater not for a moment did I have the thought, that they were actually spinning around the station much.

[lajoswinkler]

The great thing about science is that you have to provide proof. "Because I said so" isn't good enough. Care to provide evidence of the severe spinning, beyond a very biased opinion?

No severe spinning happened. My rough estimate is around 0.1 rpm.

Plug the numbers here and see for yourself.

http://www.calctool.org/CALC/phys/newtonian/centrifugal

Clooney has like 80 kg or so, based on quick Google search.

Typical EVA suit without SAFER is 115 kg. (http://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit#Specifications)

Kowalski has a powerful MMU, and typical real life MMU has 148 kg (http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit).

That's a minimum of 343 kg.

Here, let me again put this here for a reference.

Bullock's height is 1.71 m, I've found it online. If we assume we're looking at the scene from a right angle, distance between Soyuz and Stone's foot is close to 19 metres and from her hand to Kowalski is more than 5 metres. Anyhow, it's like 27 or so metres between Soyuz and Kowalski.

If you plug in the numbers and use 0.05 rpm (minimum!) you get at least 0.25 newtons of force which is a lot.

Watch 1:55, there are few seconds of the scene. It's clear the momentum is not cancelled.

I watched the movie twice, the second time carefully examining the whole issue in this scene and it was obvious they were still spinning. I think it was way more than 0.05 rpm, but regardless of that, it's not the exact numbers which are important - it's the concept. What happened in that scene is plausible and expected. Whether she'll slip or not depends on the rope-suit friction, something we don't know. But that's horrible nitpicking. The concept is valid and that's what's important.

[K^2]

The weight of a marble? Really? Was that tether coated in Teflon and grease, that she struggled to hold that weight?

Hold it against what? She wasn't attached to the rope in any way. It was freely sliding against her foot. This will provide a tiny amount of friction as you are moving away, slowing you down, but if there is a consistent force pulling you the other way, what is that going to do? The only reason she didn't slide away as well is a bit of tension built up in the rope due to the fact that they were both sliding away from the station, which allowed for a bit to recoil. And since her spacesuit was much, much lighter than Kowalski's, that was enough.

And like I said, this is an absolute minimum of the force generated. lajoswinkler goes into more detail based on footage, which raises the lower bound dramatically.

[LLlAMnYP]

Against the parachute line, of course! She was attached to it (her foot got tangled up in it). That proved to be enough to cancel both her and George Clooney's radial velocity relative to the point to which that line was attached (and as the estimates above go, that's half a metric ton at at least 1m/s). And then, all of a sudden, it is "freely sliding against her foot".

I mean, yeah, it's plausible. We could assume that at large forces the rope is tight and friction is sufficient to stop those two with a jerk (although this is the point where the 350 or so kg of George Clooney and his equipment should have just gotten ripped from her grip), but when there's only a smaller force left it starts to slip and all. All in all though, there's quite a bit of handwavium involved. Less handwavium than using a fire extinguisher to propel herself to the chinese station, but still quite a bit.

Before anyone accuses me of trolling, yes, the concept is valid, but the long and almost static scene where she is holding on to Clooney and saying "don't let go" at the very least does mislead into thinking "ok, they've stopped, now just gently pull back, your foot is safely tangled up back there". And I don't remember seeing the parachute line slipping against her leg (like I said, I'd really want to study that scene carefully again).

[K^2]

Her foot wasn't caught in it. You might be remembering wrong. The line was looped around her foot, but it was sliding pretty much freely.

[lajoswinkler]

Oh yes, it almost slipped. First almost her entire leg was caught in, the rope was over her knee, and right before Kowalski let go the tether, it was reduced down to her ankle and was just about to slip. It's one of the reasons I went to see the movie again, to confirm all this.

Also remember that in that screenshot from my last post, we see a flat 2D image of 3D space, so we can't be sure in what plane are they moving. Our vantage point is quite far away so any depth is poorly experienced.

One of the featurettes on Youtube has more of this scene.

At 03:05 you can see almost the entire scene. Most of their movement is "upwards" relative to the camera. I think it's way over 0.05 rpm.

When you see Kowalski from Stone's vantage point (not shown anywhere online, as far as I know it), you can see the stars moving behind his back.

The only implausible thing here is her being able to grab so much mass in her clunky gloves. In reality, she'd lose the grip in a snap. But hey, it's a movie. Little drama goes a long way.

Edited December 5, 2013 by lajoswinkler

[rpayne88]

*snip*

If you plug in the numbers and use 0.05 rpm (minimum!) you get at least 0.25 newtons of force which is a lot.

*snip*

How is 0.25N a lot of force? F=ma. Assuming sea level gravity of 9.8m/s^2 (32fps^2) you can divide the 0.25N of force by 9.8m/s^2 to get 0.02551kg. In other words pulling themselves up the rope would have added a mere 25g of "weight" they had to move. I am not by any means in the best shape of my life, but I can do a pull up with more than that weight. I mean, my shoes probably weigh more than that. And that is assuming you are using sea level gravity.

I haven't taken a college level physics class yet, but 25N of force doesn't seem like that much force.

[lajoswinkler]

*snip*

If you plug in the numbers and use 0.05 rpm (minimum!) you get at least 0.25 newtons of force which is a lot.

*snip*

How is 0.25N a lot of force? F=ma. Assuming sea level gravity of 9.8m/s^2 (32fps^2) you can divide the 0.25N of force by 9.8m/s^2 to get 0.02551kg. In other words pulling themselves up the rope would have added a mere 25g of "weight" they had to move. I am not by any means in the best shape of my life, but I can do a pull up with more than that weight. I mean, my shoes probably weigh more than that. And that is assuming you are using sea level gravity.

I haven't taken a college level physics class yet, but 25N of force doesn't seem like that much force.

The issue is not about Kowalski being unable to make a pull. It's about the rope which is about to slip from Stone's foot, and being unable to calculate the amount of friction, we can't claim anything except "it's plausible".

And it's way more than 0.25 N because the angular speed is much greater than estimates on this thread.

[LLlAMnYP]

-snip-

Thanks very much for this video. This actually clears a lot up. I guess, the long part with "let go - no, i won't - no you have to" kinda' blurred out the drama of this event.