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For years, Newton's third law has always bugged me. Sure, on some level I could believe it. After all, it must be true else I wouldn't be able to jump off the ground. But it always seemed extremely counterintuitive that an inanimate object with no muscles or battery or other power source could produce a very real force.

Recently, I've been revisiting the issue, and had a major revelation when reading this brilliant answer by @Rocketmagnet, which essentially describes Newton's third law as it applies to fundamental forces.

Once you accept that, for example, the repulsive forces associated with electromagnetism always come in force pairs, and that the force is inversely proportional to the (square of the) distance between two "objects", then the reactive forces we experience in our everyday lives make complete sense. Sure, my muscles may be providing the energy to drive the contact point, between my foot and the ground, closer and closer together, but that is completley incidental. What matters here is that, as a result of the decreased distance between my foot and the ground, the electromagnetic repulsive force is increased. Crucially, this force has no privileged membership to either my foot, or the ground - it is an interactive force between them.

Yet, I get the impression that when Newton's law is first taught, it is not taught in terms of (fundamental) force pairs (whose magnitude depends upon distance). Rather, it is taught in the standard "If you push on a wall, the wall pushes back". Parity at this high level description no doubt confuses many minds, as it seems to impute a physical agency to the wall. Trying to resolve this confusion by simply saying that things come in force pairs seems to be an arbitrary explanation that has no deeper meaning.

Yes, I do understand that on some level, even the deepest explanation may seem arbitrary and meaningless, but for some reason, I at least find it much easier to accept the idea of force pairs when it is presented in the context of fundamental forces and distance between objects.

So, two questions:

1) Is my understanding of the law correct?

2) Is there a reason that it is not usually taught this way?

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spacediver
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Spacediver, I DO teach physics at the high school level, so I will "take a shot" at an answer.

I have done demonstrations whereby two students interact with each other. Each student has a spring scale in their hand, and they link the two spring scales together. When each student pulls on their spring scale and reads it, they each see the same reading on their scale, indicating that the forces between them are equal in magnitude and opposite in direction. I also have had pairs of students push their hand against the hand of another student. When I ask them who is pushing, they realize that they both have to push to feel a resistance.

Often, these demonstrations do not seem to "stick" in students' minds, probably because they have misconceptions regarding how the world works. This isn't a good thing, but in the final analysis, there are some ideas in physics that just have to be accepted as facts, and I consider Newton's 3rd law to be one of those things. So to answer one of your questions, ANY TIME I apply a force to some object, there HAS to be an equal and opposite force applied to me, and that is just a fact. It doesn't matter how far apart the object is from me, as I could apply a contact force, or a magnetic force, or assuming that I was a planet (a ridiculous assumption, but bear with me) a gravity force. For all three cases, there just IS an equal and opposite force applied to me.

As a final note on this issue, I am continually amazed that when students have a misconcept that they have probably acquired from some Hollywood special effect, it is usually ALMOST impossible to get them to let go of that misconcept, and accept physical evidence that is right in front of their own eyes. If seeing is believing, and they see some physical effect with their own eyes, and STILL revert back to their misconcept on test day, it is obvious that they are not ready to think about the world around them in new and different ways.

David White
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Yet, I get the impression that when Newton's law is first taught, it is not taught in terms of (fundamental) force pairs (whose magnitude depends upon distance). Rather, it is taught in the standard "If you push on a wall, the wall pushes back".

That is the right way to teach it. Newton's third law is an abstraction. Whether it's a hammer striking a nail, the interaction between air and an airplane's wings that generates lift, or two stars interacting gravitationally, it doesn't matter how the force arises. You don't (and shouldn't) need to care what makes the force between two objects arise.

Things get even more abstract when one starts looking to the conservation laws. Newton's third law addresses the concept of "force". Even that is a bit much in the context of the conservation laws.

David Hammen
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If it helps let me put my grain of sand.

You can view Newton's 3rd law from a more general view as the statement that:

All interactions between 2 bodies must be symmetric.

If you think carefully about it, the violation of this simple and intuitive? statement would mean to see an effect over some body with minimal or no cause at all!!! (Something more like 'magic'!!!)

The causal attibute of our existence would vanish and the random chaos would reign instead. This law guarantees or rules out that scenario...

Hope this helps.

fante
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As far as I am aware, there is no theoretical proof that forces must exist in pairs. However, all fundamental forces that we are aware of exhibit this property to the most basic levels.

Newton's third law never says anything about forces existing in pairs. It says,

To every action, there must be an equal and opposite reaction

When you say your foot applies force, you are exerting electromagnetic and gravitational forces (mainly) on the ground, and the ground is doing the same. Both the foot and the ground are pushing each other. And if the ground and the foot were light years away (they can still exert forces on each other), you can talk about who pushed first!

The key mistake that you are making is your understanding that since forces here are coming in pairs, then both the foot and the ground are equivalently doing the work.

The correct answer is that it is indeed the muscles that are doing the heavy lifting, not the floor. This is because the amount of effort exerted (called work) is related to energy spent, not force.

Here's how to analyse Newtons Law of motion to the law of jumping from the ground. You coil your muscles by spending chemical energy. Then you push on the ground. By Newtons third law. The ground pushes back on your feet, your feet the rest of your body. Since your foot is stationary when in contact with the ground, the ground didn't do any work, and hence, since you spent the energy, you pushed!

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