4 ms·

I have zero issues with your logical reasoning - its the same reasoning aops gives. I'm simply asking - how do you know for sure that this actually happens in p

by dxbydt 2y ago

I have zero issues with your logical reasoning - its the same reasoning aops gives. I'm simply asking - how do you know for sure that this actually happens in practice ? Like, have we observed such a thing ? I'm genuinely curious, no snark. If all we have is reasoning, with zero observation, then its equivalent to what JFK says in Oliver Stone's picture - "Theoretical physics can prove that an elephant can hang from a cliff with its tail tied to a daisy."

lilyball 2y ago
I have no idea if we've observed it. Galaxy collisions take a very long time, I'm not sure how we'd ever be in a position to observe both the starting state and result. That said, the point of the law of conservation of momentum is we don't need to observe it to know that it happens. Momentum is conserved, that's a fact. So the question becomes, what is it that makes this law not produce a good prediction about a scenario? And the answer is that when the scenario involves other factors that can take kinetic energy away from the system. In the car & truck scenario, we have friction with the road as a pretty huge factor, that removes a lot of energy. Even without a collision, friction is why a car needs to constantly burn fuel in order to keep moving at the same speed. So it should be no surprise that a car & truck collision will lose a ton of energy to friction. In the eagle landing scenario, that's not even a simple collision between the eagle and the branch, the eagle uses its wings to slow down, and the branch is fixed to a tree and absorbs the remaining energy. An eagle landing on a branch is an eagle that comes to rest on the branch, it doesn't just not conserve momentum it gets rid of all of it. But in the colliding galaxies scenario, there's no surface to have friction with, there's no wings beating against air, there's no tree anchoring one of the objects in place, nothing to absorb any energy, no environment to dissipate energy in. Without anything to get rid of kinetic energy, conservation of momentum will predict the results.
- saagarjha 2y ago
  That is, until gravitation waves carry away the energy :)
  - trashtester 2y ago
    It doesn't matter much if they carry away energy, though the waves also carry away some momentum, I suppose, unless they're symmetric on both sides.
ninetyninenine 2y ago
The question is testing if your reasoning is in line with what you were taught. It is not questioning your ability to validate your reasoning experimentally. I came to the same conclusion as the replier almost immediately as I was reading the question. It was obvious to me. But I do agree it’s a bad question as it relies on a lot of assumptions. Maybe the question is just testing if your ability to make reasonable estimates assumptions.

by dxbydt 2y ago

lilyball 2y ago

I have no idea if we've observed it. Galaxy collisions take a very long time, I'm not sure how we'd ever be in a position to observe both the starting state and result. That said, the point of the law of conservation of momentum is we don't need to observe it to know that it happens. Momentum is conserved, that's a fact. So the question becomes, what is it that makes this law not produce a good prediction about a scenario? And the answer is that when the scenario involves other factors that can take kinetic energy away from the system. In the car & truck scenario, we have friction with the road as a pretty huge factor, that removes a lot of energy. Even without a collision, friction is why a car needs to constantly burn fuel in order to keep moving at the same speed. So it should be no surprise that a car & truck collision will lose a ton of energy to friction. In the eagle landing scenario, that's not even a simple collision between the eagle and the branch, the eagle uses its wings to slow down, and the branch is fixed to a tree and absorbs the remaining energy. An eagle landing on a branch is an eagle that comes to rest on the branch, it doesn't just not conserve momentum it gets rid of all of it. But in the colliding galaxies scenario, there's no surface to have friction with, there's no wings beating against air, there's no tree anchoring one of the objects in place, nothing to absorb any energy, no environment to dissipate energy in. Without anything to get rid of kinetic energy, conservation of momentum will predict the results.

saagarjha 2y ago
That is, until gravitation waves carry away the energy :)
- trashtester 2y ago
  It doesn't matter much if they carry away energy, though the waves also carry away some momentum, I suppose, unless they're symmetric on both sides.

ninetyninenine 2y ago

The question is testing if your reasoning is in line with what you were taught. It is not questioning your ability to validate your reasoning experimentally. I came to the same conclusion as the replier almost immediately as I was reading the question. It was obvious to me. But I do agree it’s a bad question as it relies on a lot of assumptions. Maybe the question is just testing if your ability to make reasonable estimates assumptions.