Question about the tether scene from the movie Gravity

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fereopk
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Question about the tether scene from the movie "Gravity"

In the movie there's a scene where two astronauts are joined together by a rope. One is accelerating to pull the other. The astronaut would accelerate, the rope would tense and give a jerk to the other astronaut. Then the rope would loosen again until the accelerating astronaut gained enough velocity to tense the rope again, giving another jerk.

If something had very little mass, I believe the rope would simply remain tense. My question is, how do you determine if the rope remains tense or loosens? How much more velocity does the astronaut being pulled gain over the accelerating astronaut such that they loosen the rope?
 

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  • #2
Andrew Mason
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In the movie there's a scene where two astronauts are joined together by a rope. One is accelerating to pull the other. The astronaut would accelerate, the rope would tense and give a jerk to the other astronaut. Then the rope would loosen again until the accelerating astronaut gained enough velocity to tense the rope again, giving another jerk.

If something had very little mass, I believe the rope would simply remain tense. My question is, how do you determine if the rope remains tense or loosens? How much more velocity does the astronaut being pulled gain over the accelerating astronaut such that they loosen the rope?
I haven't seen the movie. But if there are no external forces acting on either astronaut (there is just the tension in the rope), then by Newton's third law, the tension in the tether must result in equal and opposite forces (ie. the tension) being applied to both astronauts for the same time. So, if one astronaut pulls on the rope, absent any forces other than the pull on the rope, the two astronauts to move toward each other and keep moving together until they collide. When they collide, if one or both of the astronauts push off against the other, the two astronauts will move apart until the rope becomes tight and stops them.

AM
 
  • #3
256bits
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Think stretchy rope like a bungee cord.
 
  • #4
CWatters
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As I see it you have two astronauts initially moving apart. I suspect that when the rope tightens it acts like a spring so there will be an elastic collision. What happens next depends on the relative masses and velocities (any damping/shock absorbing properties of the rope?)
 
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fereopk
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I haven't seen the movie. But if there are no external forces acting on either astronaut (there is just the tension in the rope), then by Newton's third law, the tension in the tether must result in equal and opposite forces (ie. the tension) being applied to both astronauts for the same time. So, if one astronaut pulls on the rope, absent any forces other than the pull on the rope, the two astronauts to move toward each other and keep moving together until they collide. When they collide, if one or both of the astronauts push off against the other, the two astronauts will move apart until the rope becomes tight and stops them.

AM

One of the astronauts is actually continually accelerating using his booster to pull the other. When he gets far enough, the rope tenses and then de-tenses, pulling the other astronaut.
 
  • #6
phinds
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There were plenty of technical flaws in the movie but the tether action was not among them.
 
  • #7
A.T.
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One of the astronauts is actually continually accelerating using his booster to pull the other. When he gets far enough, the rope tenses and then de-tenses, pulling the other astronaut.
If he would accelerate under constant thrust for a while, the rope would stay tense at some point.
 
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fereopk
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There were plenty of technical flaws in the movie but the tether action was not among them.

Yes I was just looking for an explanation.
 
  • #9
fereopk
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If he would accelerate under constant thrust for a while, the rope would stay tense at some point.

If he was tethered to something with very little mass, like a paperclip, and started accelerating, intuitively the rope would just remain tense, right? But if it was something like a planet, there would be a sort of jerk that would pull him back, loosening the rope? How do you know what will happen?
 
  • #10
A.T.
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If he was tethered to something with very little mass, like a paperclip, and started accelerating, intuitively the rope would just remain tense, right? But if it was something like a planet, there would be a sort of jerk that would pull him back, loosening the rope? How do you know what will happen?
If the connection is perfectly elastic there will be endless oscillations. If the connection is perfectly inelastic there will be no oscillation.
 
  • #11
A.T.
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In the movie there's a scene where two astronauts are joined together by a rope. One is accelerating to pull the other. The astronaut would accelerate, the rope would tense and give a jerk to the other astronaut. Then the rope would loosen again until the accelerating astronaut gained enough velocity to tense the rope again, giving another jerk.
Instead of jerking around, wouldn't it make more sense to pull the rope, come together, grab each other and then use the booster?
 
  • #12
fereopk
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If the connection is perfectly elastic there will be endless oscillations. If the connection is perfectly inelastic there will be no oscillation.

It was very in-elastic, there was almost no stretching. Even if there was stretching, wouldn't it remain tense as long as the constant acceleration is applied? Why would it oscillate if the same force that brought it to that stretch is still there?
 
  • #13
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It was very in-elastic, there was almost no stretching. Even if there was stretching, wouldn't it remain tense as long as the constant acceleration is applied? Why would it oscillate if the same force that brought it to that stretch is still there?

If there's any stretching at all, then disturbances at one end of the tether will take some non-zero time to propagate to the other end and you'll get some sort of wave behavior; for example, it's surprisingly hard to swing an object by an elastic band while maintaining a fixed extension of the band.

The intensity of this effect will depend om many factors: How much energy is dissipated in internal friction within the tether; the mass of the tether; the elasticity of the tether; and how quickly and smoothly the force is applied.
 
  • #14
A.T.
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Seems like the biggest challenge would be avoiding spinning.
 
  • #15
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There would be tension on the tether - they got it almost right (although I think the tension was exaggerated - and wrong direction? not sure). Why? Assume they are static, no stretch rope or wave theory. Assume the station was in a stable orbit. An object is in orbit when the centrifugal force acting on it is equal and opposite to the gravity it's subjected to. That's why a person in orbit feels truly weightless. Our heroes were traveling at the same speed as the ISS, but were at a different altitude (by the length of the tether & rope), and thus there would be a slight difference between the centrifugal and gravitational accelerations - therefore they would drift apart.

I'm pretty sure George would have to let go, else we wouldn't have been treated to the 'sunrise on the Ganji' line.
 
  • #16
Andrew Mason
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There would be tension on the tether - they got it almost right (although I think the tension was exaggerated - and wrong direction? not sure). Why? Assume they are static, no stretch rope or wave theory. Assume the station was in a stable orbit. An object is in orbit when the centrifugal force acting on it is equal and opposite to the gravity it's subjected to. That's why a person in orbit feels truly weightless.
Welcome to PF horrido.

The Newtonian forces on an orbiting body are not balanced. Since the centrifugal force is not a real Newtonian force it cannot balance gravity.

A person free-falling straight down to the earth through space would feel the same thing (ie. until they hit the atmosphere). So it is not because of rotation or because the forces are balanced that the person feels weightless. It is the fact that all parts are accelerating at the same rate (so that there are no perceptible tension within the astronaut's body resulting from this acceleration) that creates the feeling of weightlessness. This applies to any body on whom the only forces acting are gravitational forces that do not change materially over the entire body.

AM
 
  • #17
sophiecentaur
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This tether problem is not only relevant in space. Using a rope to tow another car can present big problems if the driver of either car is heavy footed on brake, clutch or accelerator. It may not be very exotic but it's a problem that any of us might find ourselves landed with.
I think I'd rather be tethered to Sandra Bullock, though.
 

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