Timeline for Action/reaction pair, 3rd law
Current License: CC BY-SA 4.0
15 events
| when toggle format | what | by | license | comment | |
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| Jun 6, 2023 at 0:48 | comment | added | Amit | Correction: Ideally, they're only in "perfect" balance once the new equilibrium is found, again like increasing the force on a spring, it will get compressed only to the displacement where it can react with an equal and opposite force. But for a few moments they are not in balance, that's what's missing from the diagram (/ animations too probably) -- displaying this small time scale when the forces are out of balance. | |
| Jun 6, 2023 at 0:41 | comment | added | Amit | They're never in perfect balance because this is in fact a dynamical system! as the text in the diagram also points out a bit like how a spring takes time to react, only here it happens much quicker so you may be confused by the static-looking picture. The wall gets compressed a bit, like a spring, and during the compression of the spring the forces are not balanced, there is a net force to the right! A few milliseconds, say, after that, the wall (spring) reacts with an equal and opposite force, but by that time the wall (spring) is already a bit compressed, so a new equilibrium is reached. | |
| Jun 6, 2023 at 0:26 | comment | added | drzaius7 | I'm not sure I follow you. I'm saying that red should always equal green. But the text I screenshotted says "if the [green] force is too small to balance the pushing [red] force, the box will continue to move right." How can they ever be out of balance is my question. | |
| Jun 6, 2023 at 0:17 | comment | added | Amit | Now the next thing to realize, N3L always applies, even if the wall had a finite mass and not an infinite one as is idealized here. The compression of the wall is simply a slight relaxation of that, in admitting that at least the surface of the wall can be imagined as being made of many little molecules each with very small mass, and they will be accelerated due to the box's force and hence the wall's surface compressed. If the analysis was more complete, it would also show a diagram of a few molecules near the surface of the wall and force vector from the box's surface being applied to them | |
| Jun 6, 2023 at 0:16 | comment | added | drzaius7 | I understand all of that. my confusion stems from the chain of reasoning I laid out. specifically: blue = green (by N3L) and red = blue. Given those two relations, how can red ever NOT equal green (at least in magnitude)? | |
| Jun 6, 2023 at 0:00 | comment | added | Amit | I think I see the difficulty then: N3L applies to the blue and green forces because the wall applies an equal and opposite force to the box, as the box applies to it. Note, both forces are acting on different objects. Now, the red force is postulated to appear "magically" out of nowhere, just as a proxy in place of some agency that's pushing on the box! You can imagine, that it is in fact a person doing this pushing. Then, there will also be an action reaction pair in accordance with N3L! The box will push back on the person with an equal and opposite force. | |
| Jun 5, 2023 at 23:56 | comment | added | drzaius7 | I think my difficulty lies in the idea that the red force would ever NOT be in balance with the green force given blue = green (by N3L) and red = blue. One thought is that maybe the imbalance gets created by the elasticity of the wall; part of the green force is getting temporarily "absorbed" until it reaches a certain threshold of compression, at which point the stored up force is released. | |
| Jun 5, 2023 at 23:29 | comment | added | Amit | I'm not 100% sure what is your difficulty, but I have a half educated guess that maybe this post can be of help! | |
| Jun 5, 2023 at 22:48 | history | edited | Qmechanic♦ | CC BY-SA 4.0 |
edited tags
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| Jun 5, 2023 at 22:28 | answer | added | Tru Physics | timeline score: 0 | |
| Jun 5, 2023 at 21:50 | answer | added | Chet Miller | timeline score: 1 | |
| Jun 5, 2023 at 21:37 | answer | added | Claudio Saspinski | timeline score: 0 | |
| Jun 5, 2023 at 19:33 | answer | added | librasteve | timeline score: -5 | |
| S Jun 5, 2023 at 19:22 | review | First questions | |||
| Jun 5, 2023 at 19:39 | |||||
| S Jun 5, 2023 at 19:22 | history | asked | drzaius7 | CC BY-SA 4.0 |