Why they turn off the Reaction force?

Hi, I am very confused by the definition of reaction force. Even I read the official tutorials again and again. I want to know the physical meaning of reaction force. Is there any case that there is no reaction force in our daily life? For example. if I push the door, it will give me a reaction force that can push me back. In what condition this reaction force doesn't exist?
Another question is in the demo of Bench Press, what's the physical meaning of AnyKinEq WeightRotFix? What is its axis of rotation? Why do we need to turn off the reaction force in the Y and Z axis directions?

Thank you!

Hi @Taejun

Here is some info on reactions.

The AnyKinEq is a kinematic equation - a constraint on a kinematic measure.
In you example is specifies a constraint on the rotation of the weight segment.
The Reaction.Type specifies if the internal AnyReacForce is turned on/off for each direction of the measure (x,y,z). So for your example there is a reaction force on the x-rotation of the weight, meaning the human model does not need to exert muscle force to "carry" that degree of freedom. It is neglected.

Best regards,

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