# Joint Actuation Inputs: Specify both Motion and Force/Torque

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Nicholas Zahnweh on 12 Jan 2021
Answered: Yifeng Tang on 29 Apr 2021
Hey all,
why is it possible to specify both the Motion and the Force (or Torque, depending on the joint primitive) as "Provided by Input". I´ve read the documentation (de.mathworks.com/help/releases/R2017a/physmod/sm/ug/joint-actuation.html) regarding that topic, it´s also stated in there that this is possible (it´called in there "fully computed").
The same problem understanding arises by specifying both with "automatically computed".
Could somebody please explain in which scenario that would be useful and what is internally happening?
Best regards
Nicholas

Yifeng Tang on 29 Apr 2021
Hi Nicholas,
It is possible to specify EITHER motion OR force as "provided by input", but not both. Specifying both is physically unreasonable and I believe you'll get an error when trying to simulate the model.
So the common combinations are:
1. Force=None, Motion=Auto: just a regular joint, constraining the motion by the joint definition and won't apply any additional actuation effort.
2. Force=Input, Motion=Auto: applying specified force at the joint, motion is solved based on the system of equations.
3. Force=Auto, Motion=Input: applying prescribed motion (usually needs some derivatives), force is calculated and is the amount of force needed to generate such joint motion
The other combinations:
1. Force=None, Motion=Input: not sure what will happen, probably will throw an error.
2. Force=Input, Motion=Input: error.
3. Force=Auto, Motion=Auto: not sure, likely error, under-constraint system I suppose.
Hope this helps.

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