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: 
What needs to be incorporated into this picture, is that a walking human in this context cannot be approximated by a point-like object - the approximation that we commonly make in introductory mechanics. Walking implies forces applied between different parts of the body, notable between the legs and the torso, as well as the forces acting between the feet and the ground, and the gravity.

Thus, the force that makes us move forward is not the friction force, but the force exerted by our muscles, making the parts of our body move in respect to our foot, held in place by a friction force. The foot meanwhile remains static - they do not accelerate.

The ankle exerts a force on the foot, and the foot exerts a force on the ground, which is counteracted by the friction force. If the static friction force is not able to hold the foot in place, as it happens when walking on ice, the foot will slip and accelerated by the ankle force.
* While walking there is another force acting which is force due to gravity.
* If there is not enough friction your foot will keep traveling backwards instead of staying put and gravitational force will pull the body down (slipping).