Maximum Steelpush Destruction

[therunner he/him]

3 hours ago, Duxredux said:

Uh... I'm self-conscious about explaining physics when there's a literal professor on the thread, but aren't you ignoring the normal force in your explanations? The only difference between abruptly thrusting your arms forward and a pushup is the normal force. In the case of Vin and the coin, pushing with the coin is the equivalent of pushing out with a stick and the stick running into an unexpected wall but with way more force. Force is added into the equation when the coin or stick hits the wall via the normal force even though the force of the push remains constant. In Vin's case the amount of force she was applying to the coin would have been sufficient to throw her to the side had the coin started out pressed against the wall.

My terminology is rusty, but it feels like you're conflating force with another concept, maybe work.

No the difference is that you don't actually apply the same force to the air as you do the the wall/ground.

But with Vin/Mistborn she is applying the exact same force to the coin, so she should experience the very same reaction whether the coin hit the wall or not. Since that is not what we see, 3rd Newton law is not applicable for the situation.

The key difference is that Vin is always pushing on the coin with equal force, but the person is pushing against two different things with two different forces (air/wall).

[Frustration he/him]

12 hours ago, therunner said:

No the difference is that you don't actually apply the same force to the air as you do the the wall/ground.

But with Vin/Mistborn she is applying the exact same force to the coin, so she should experience the very same reaction whether the coin hit the wall or not. Since that is not what we see, 3rd Newton law is not applicable for the situation.

The key difference is that Vin is always pushing on the coin with equal force, but the person is pushing against two different things with two different forces (air/wall).

I'm not understanding how pushing a wall suddenly generates extra force, could you please explain that?

[therunner he/him]

12 hours ago, Frustration said:

I'm not understanding how pushing a wall suddenly generates extra force, could you please explain that?

It does not.

The point is that you don't shove air with the same force you would shove the wall, so you don't experience the same pushback.

[Argenti he/him]

6 hours ago, therunner said:

The point is that you don't shove air with the same force you would shove the wall, so you don't experience the same pushback.

Why wouldn't you?  You don't push on the wall; you push on the coin. The coin "wants" to keep moving foward and if it can't, due to a wall or other large object, the force will go somewhere and push the allomancer back. Think about a push like a spring, you push down and get the same force back, but only if it hits something and can't go there. 
 The force doesn't change, the resistance to the force does. it's normal force.

[therunner he/him]

1 hour ago, Argenti said:

Why wouldn't you?  You don't push on the wall; you push on the coin. The coin "wants" to keep moving foward and if it can't, due to a wall or other large object, the force will go somewhere and push the allomancer back. Think about a push like a spring, you push down and get the same force back, but only if it hits something and can't go there.

...That is not how Newton's 3rd Law works.

There is always equal and opposite force acting back on you. If Vin is pushing the coin with force F, and we assume Newton 3rd Law holds, then the coin pushes back on Vin with force -F. Wall or no wall.
That is why that particular scenario does not work.

[Argenti he/him]

8 minutes ago, therunner said:

...That is not how Newton's 3rd Law works.

There is always equal and opposite force acting back on you. If Vin is pushing the coin with force F, and we assume Newton 3rd Law holds, then the coin pushes back on Vin with force -F. Wall or no wall.
That is why that particular scenario does not work.

Stolen from Quora

Quote

The action and reaction in Newton's law act on different objects. You push the air and exert a certain force on the air; the air pushes back on your hand with equal force - equal to the force you exert on the air, not equal to the force your arm muscles exert on your hand to make it move.

--

Compare the following two statements:

1. If object A acts on object B by applying a force, object B reacts by exerting a force on object A with the same magnitude and opposite direction.

2. Whenever a force is applied to object X, another force reacts on object X with the same magnitude and opposite direction.

The first statement is Newton's third law. The second is fiction, and it's the one which - if it were true - would force everything to not ever move (more precisely, nothing would ever accelerate, so if anything was at rest, it would stay at rest forever).

I think the second version is what the OP thinks Newton's third law is saying, but it doesn't.

But beyond that, if you want all the physics you could ever want go here.

 

[alder24]

28 minutes ago, Argenti said:

Stolen from Quora

But beyond that, if you want all the physics you could ever want go here.

I think what @therunner tries to say is that the force of steelpush acting on a coin is the same, no matter if it flies through the air, or it hits a wall - we can agree on that one. Then, because of Newton's 3rd Law, the force of reaction would act on Vin continuously when she is pushing the coin. This force is equal to the force of Vin's steelpush, and because of Newton's 3rd Law it doesn't matter if the coin is in the air or it hits a wall, the same force is still applied to Vin. Therefore it makes no sense for Vin to fly backwards when the coin hits a wall, because the force acting on her didn't change, it should be the same as it was when the coin was flying through the air. But we clearly know that a Mistborn is flying when coins hit the ground/wall, thus Newton's 3rd Law doesn't apply here, as another force starts acting on her the moment the coin hits a wall and that's not the force of reaction from Newton's 3rd Law.

To say it simply, Newton's 3rd Law can't be the reason why Mistborn are flying when coins hit the ground, because the force of reaction didn't change and is the same as when coins were flying through the air before they hit the wall.

[Argenti he/him]

2 minutes ago, alder24 said:

I think what @therunner tries to say is that the force of steelpush acting on a coin is the same, no matter if it flies through the air, or it hits a wall - we can agree on that one. Then, because of Newton's 3rd Law, the force of reaction would act on Vin continuously when she is pushing the coin. This force is equal to the force of Vin's steelpush, and because of Newton's 3rd Law it doesn't matter if the coin is in the air or it hits a wall, the same force is still applied to Vin. Therefore it makes no sense for Vin to fly backwards when the coin hits a wall, because the force acting on her didn't change, it should be the same as it was when the coin was flying through the air. But we clearly know that a Mistborn is flying when coins hit the ground/wall, thus Newton's 3rd Law doesn't apply here, as another force starts acting on her the moment the coin hits a wall and that's not the force of reaction from Newton's 3rd Law.

To say it simply, Newton's 3rd Law can't be the reason why Mistborn are flying when coins hit the ground, because the force of reaction didn't change and is the same as when coins were flying through the air before they hit the wall.

ohhhhhh 

[therunner he/him]

Thank you @alder24 for explaining it better than I did!
That is indeed what I meant

[DrPhysics he/him]

On 10/25/2023 at 9:59 PM, Duxredux said:

Uh... I'm self-conscious about explaining physics when there's a literal professor on the thread

Don't be. That's how we learn physics - try to use it to explain the world, and then fix our mistakes when we are wrong. No one learns physics by sitting and listening to a teacher. (We've been studying physics education for 50 years, and that is one consistent theme that pops up over and over again.)

And in this thread, everyone got to the right place in the end. That's what really matters.

Also, Newton's third law is really hit or miss in the first Mistborn trilogy. Based on interviews and Era 2, it's obvious that it is supposed to apply, but Newton's first and third laws are both really hard to wrap your head around and so there are lots of situations where one or both get broken unless you jump through some serious hoops to get them to work.

Edited October 27, 2023 by DrPhysics
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