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The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else. The other particle might be far away, as with the earth–sun system, but...

posted 2y ago by Olin Lathrop‭  ·  edited 2y ago by Olin Lathrop‭

#2: Post edited by Olin Lathrop‭ · 2021-08-14T17:39:15Z (about 2 years ago) copy link
• <blockquote>The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else. The other particle might be far away, as with the earth–sun system, but it’s always out there somewhere.</blockquote>
• This was probably embedded in more context. It seems the point he is trying to make is that for one object to accelerate, it must push on some other object. Therefore, that other object must also be accelerating.
• When a rocket is accelerating in space, it causes propellant to accelerate in the opposite direction. When you start running on earth, you push on the planet, which accelerates very slightly in the opposite direction. Even if you're using a solar sail in space, the photons hitting the sail accelerate in the opposite direction. Since photons have finite energy, they have finite momentum. The momentum is being swapped between the photons and the sail to push the spacecraft.
• In all the examples above, the total momentum was conserved if you examine a bubble large enough to encompass both objects completely.
• Another way to paraphrase this is that you can't accelerate without pushing on something else. That something else will therefore also accelerate, although in the opposite direction. The total momentum is conserved.
• <blockquote>The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else. The other particle might be far away, as with the earth–sun system, but it’s always out there somewhere.</blockquote>
• This was probably embedded in more context. It seems the point he is trying to make is that for one object to accelerate, it must push on some other object. Therefore, that other object must also be accelerating.
• When a rocket is accelerating in space, it causes propellant to accelerate in the opposite direction. When you start running on earth, you push on the planet, which accelerates very slightly in the opposite direction. Even if you're using a solar sail in space, the photons hitting the sail accelerate in the opposite direction. Since photons have finite energy, they have finite momentum. The momentum is being swapped between the photons and the sail to push the spacecraft.
• In all the examples above, the total momentum was conserved if you examine a bubble large enough to encompass both objects completely.
• Another way to paraphrase this is that you can't accelerate without pushing on something else. That something else will therefore also accelerate, although in the opposite direction. The total momentum is conserved.
• <hr>
• <blockquote>Olin photons can't accelerate like most of the objects because photons always move with one speed for all inertial frames.</blockquote>
• I was over-simplifying because this level of detail was not relevant to the question. Yes, photons don't change their speed, but they still have momentum that gets transferred to any object they bounce off of or are absorbed by.
• Their speed may not change, but their velocity (speed in a specific direction) certainly can. You can use that change in velocity to compute an effective acceleration, even if the speed doesn't change.
• Hopefully this digression didn't confuse the point of the OP's actual question too much.
#1: Initial revision by Olin Lathrop‭ · 2021-08-14T13:58:09Z (about 2 years ago) copy link
<blockquote>The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else. The other particle might be far away, as with the earth–sun system, but it’s always out there somewhere.</blockquote>

This was probably embedded in more context.  It seems the point he is trying to make is that for one object to accelerate, it must push on some other object.  Therefore, that other object must also be accelerating.

When a rocket is accelerating in space, it causes propellant to accelerate in the opposite direction.  When you start running on earth, you push on the planet, which accelerates very slightly in the opposite direction.  Even if you're using a solar sail in space, the photons hitting the sail accelerate in the opposite direction.  Since photons have finite energy, they have finite momentum.  The momentum is being swapped between the photons and the sail to push the spacecraft.

In all the examples above, the total momentum was conserved if you examine a bubble large enough to encompass both objects completely.

Another way to paraphrase this is that you can't accelerate without pushing on something else.  That something else will therefore also accelerate, although in the opposite direction.  The total momentum is conserved.