Question: I have to find the Norton equivalent for the circuit external to $ R\_2 $ (not sure if that's the way to say it, I'm translating from Spanish). The problem is that I don't even understand what's going on in the diagram.

[![Circuit](https://i.stack.imgur.com/AXUqx.png)](https://i.stack.imgur.com/AXUqx.png)

Are $ E\_1 = + 12V $ and $ E\_2 = -4 V $ different ends of a single voltage source or two separate ones? And in any case, where do I place the "open terminals" of the circuit when I remove $ R\_2 $ ?

Answer: 
Your leg muscles apply a force to your foot, relative to the rest of your body. If your are on normal ground, the ground applies a force through friction to your foot. As a result, there is not net force, and your foot acts as an intermediary that allows your body to apply a force to the ground, and as a result there is a reaction force in which the ground applies a force to your body. Your foot remains stationary while the rest of your body moves forward. Walking consists of alternating which foot remains stationary, so that overall each foot moves with the rest of your body.

If you were to apply the same force to your foot while standing on a frictionless surface[1], there would be no (horizontal) force external to your body, so your center of mass would remain stationary. This would mean that your foot would move backwards, and the rest of your body would move forward slightly.

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> If I understand when walking, the road will have some friction and if I exert a backwards force against the friction of the ground
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You are not exerting a force on the ground directly. You are exerting a force on your foot, the ground is exerting a force on your foot, and the force of the ground on your foot is accompanied by a force that your foot exerts on the ground.

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> But where does my force go? It makes me accelerate?
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With no external forces, your body as a whole cannot accelerate. You can only have acceleration of one part of your body relative to the rest. Without the force of the ground to counteract the force of your muscles, your foot accelerates backwards, but the rest of your body has a forward force that matches the backwards force on your foot.

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> I don't think "slipping" means accelerate.
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Slipping means moving past. Normally when you walk, at each moment there is at least one foot on the ground that is stationary. If you're slipping, that means that the foot on the ground is moving. Moving is not itself acceleration, but it is the result of acceleration.

[1]As others have pointed out, friction is really a property of the two substances in contact, not either individually, so a more precise phrasing would be "a surface that does not have any friction between in and your foot", but it's simpler to say "frictionless surface".
When we walk or run we apply a pushing force against the ground. The ground applies an equal and opposite reaction force on us.

. During walking or running the normal reaction force is greater than the gravitational force on the person in order to lift the person off the ground.

The static friction force that the ground applies to the person helps forward motion, and is equal and opposite to the parallel force the person applies to the ground. If there were no static friction force the person will slip.

`If my "force" is bigger than the frictional force then the friction won't be able to provide that force in the opposite direction, so my foot will "continue its path" i.e. slip. But where does my force go? It makes me accelerate? I don't think "slipping" means accelerate.`

If friction not sufficient enough then changes from static to kinetic friction.this value is less than the force you apply I.e the muscular force so though there is an opposition the opposition is just not enough to make you stop. In a way you will be accelerating

$a$=$(F-f\_r)$/m
In short your force is causing your acceleration