Talk:Reaction (physics)

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Here is a complete re-write of this article. I have removed the stub pointer. Comments and suggestions are welcome. --Michel M Verstraete 22:24, 21 June 2006 (UTC)

This is an excellent article, I must particularly thank you for concentrating on the common misunderstandings of Newton's Third Law, as I suffer from them myself. I agree, I think that teachers and writers usually express it too curtly, in ways that allow incorrect ideas, and, worse, they don't take care to steer people away from those incorrect ideas. When people then go away and try to think about the Third Law they sooner or later trip up over contradictions and end up thinking they cannot understand the Third Law.

I wonder if you could add to your "Examples of common misunderstandings" a case that has long puzzled me: if a car drives through a brick wall, the car has applied a force to the wall, but the wall seems not to have applied an equal force back on the car, because the wall has collapsed. Stephen Fennell 217.33.113.3 10:09, 17 May 2007 (UTC) .

Dear Stephen, Thanks for your kind words, and apologies for the delayed response to your question: I only just saw your comment today.

The car crashing into a wall is subject to the same laws of physics as all other objects of the Universe, but this case is rather more complicated, because it involves not just objects moving with respect to each other but also deforming (to the point of breakdown). The simple answer is that the car will likely be destroyed too (or at least severely dented) in this experiment. If you replace the car by a cyclist, you will see that in that case the wall stands and the cyclist is hurt: it thus has to do with the strength or solidity of the objects that interact. The fact that the wall collapses does not imply it has no impact on the car.

A more detailed answer would go along the following lines: As soon as the car bumper touches the wall, it exerts a pressure force on the wall, and the wall exerts a similar (reaction) pressure force on the car. As long as these forces remain 'small enough', they only result in elastic deformations of both objects, following [Hooke's law] (Visualize a plastic bumper and a carton wall, for instance). As the pressure from the car increases, the reaction from the wall also increases. At some point, the structure of some of the materials involved will break down, the interaction becomes [Elasticity (physics)|inelastic], and the weaker one looses its ability to hold itself in one piece and thus also its ability to react. At that point, the car does not apply any force on the wall anymore (the contact point or surface has vanished) and thus the wall does not exert a force on the car either. The fact that the wall collapses is a subsequent consequence of the structural instability of the wall itself, following the piercing of a large hole and/or the shock wave induced into the wall; this has little or nothing to do with the car, as far as the action-reaction law is concerned.

I hope this helps. Michel M Verstraete 22:50, 5 June 2007 (UTC).

[edit] Action and Reaction Must be of Same Physical Nature?

The article currently has a paragraph that reads:

Another important point to keep in mind is that the physical nature of the reaction force is identical to that of the action itself: if the action is due to gravity, the reaction is also due to gravity. Hence, any discussion of this topic that amounts to a claim that an action results in a reaction of a different type (gravitational, electromagnetic, friction, spring, or whatever) is obviously wrong and should be discarded.

I've never heard about this. Can we have a source for it? Hairouna 04:13, 12 September 2007 (UTC)

Think of it this way: While the definition of a force involves an accelerating mass, this thing does not normally exist in isolation. So if one billiard ball strikes another, at the moment of impact there is a force affecting BOTH balls. The simplest way to generically say it is, "A force typically comes into existence between two interacting things. It is the interaction that yields the force, and simply because both things are involved in a particular type of interaction, both things experience the same type of force.

All that said, I'm not sure I can agree with the claim that the preceding is always true for every possible kind of force. For example, an electron can absorb a photon, and acquire the kinetic energy and momentum of the photon. WHEN THIS EVENT happens, DURING it, the photon disappears and the electron experiences a force that accelerates it to a new velocity. I acknowledge this event properly belongs to the realm of Quantum Mechanics, where strange things are allowed (the electron may be described as literally instantly starting to move at the new velocity, and no Newtonian-type of acceleration/force may be involved at all). Nevertheless, this event does pose a problem with respect to the notion of Action and Reaction, simply because, while obviously the electron is Acting, after absorbing the photon, nothing exists that is Reacting! —Preceding unsigned comment added by 216.9.73.2 (talk) 08:19, 7 January 2008 (UTC)

[edit] Book on the table

I've edited the explanation of one of the misunderstandings, to highlight where the confusion comes from. The force exerted by the table can be rightfully called reaction (and indeed that is often the case, as in ground reaction), as long as it's clear what force it is a reaction to. Also, I don't see why the table and the book are not at rest (in any inertial frame of reference). I think the statement implicitly assumes the Earth as reference frame, neglecting any effects due to its rotation, therefore both book and table are at rest, with respect to it. Anyway, the at-rest question is non-relevant to the 'weight-action/table-reaction' confusion and so I've removed it.
Giuliopp (talk) 19:25, 17 November 2007 (UTC)

A new misunderstanding has been added. One way to test the description involves the toy Newton's cradle. It usually has about 5 suspended balls. Why do you never see a version of that toy with, say, 200 balls? (If you worry about accurate alignment, just replace all the center balls with a simple rod.) The answer relates to the time it takes the mechanical force to propagate to the last ball in the row. When the distance between first and last ball is short, the system allows essentially all the momentum to transfer easily from the first ball to the last ball. But when the distance is long, the first ball partly bounces back, and the last ball doesn't swing out as far as it does when the distance is short. The row of balls is acting like a massive solid object, NOT INSTANTLY RESPONDING AS A WHOLE TO THE APPLIED FORCE, until the last ball starts to move, and it is that "acting like a solid object" that causes the first ball to bounce. In this case, then, the action/reaction of the impact force between the first ball and the row occurs simultaneously, but the actual movement-action/reaction of the bodies is non-simultaneous, exactly as is described in the added "misunderstanding" text. —Preceding unsigned comment added by 216.9.73.101 (talk) 23:30, 1 January 2008 (UTC)

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