# Mechanical Analogies: Speed & Force

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Hi everyone,

after watching the physical domain introduction webinar, a question popped up in my mind.

If you look at the table of domain analogies, Electrical Voltage equals Mechanical Speed while Current equals Force/Torque.

Is there a specific reason why it was chosen this way round? I always thought it to be much more intuitive to pair Voltage with Force/Torque and Current with (angular) speed.

Reason:

Current "flows", i.e. moves -> much like speed.

An inductor keeps current flowing, just like inertia keeps a mass moving.

A capacitor holds static charge. A static form of mechanical Energy storage is a spring, hence voltage would be paired with force and so on.

Now why has it been chosen the other way round? There must be a reason, I guess? Is it easier to solve some equations this way round?
asked Jul 11, 2018

This is best explained with an example such as the one below. It shows the mechanical model of two masses coupled by a spring and damper and its equivalent electrical circuit using the PLECS analogies:

Now try and draw the electrical circuit using your proposed analogies. (It is certainly possible, but the circuit won't look very similar to the mechanical model.)
answered Jul 12, 2018 by (401 points)

Hi Wolfgang,

thanks for taking the time to answer the question. It was a good example to show the differences in the analogies.

In fact, it inspired me to draw up my proposed analogy.

I see how the spring-damper system looks more similar to the elctrical counterparts with PLECS's analogies. Mine on the other hand show a more intuitive usage of masses (inductors) and I prefer the analogies of speed->current / voltage->force.

But at least now I understand where PLEXIM is coming from and also why they made their decisions the way they are - personal preference.

Thanks!