# Calculating the resultant forces via the pressure and shear stress terms in Navier-Stokes

**URL:** <https://community.freefem.org/t/calculating-the-resultant-forces-via-the-pressure-and-shear-stress-terms-in-navier-stokes/2107>\
**Category:** General Discussion\
**Created:** [November 7, 2022, 9:33am UTC](https://community.freefem.org/t/calculating-the-resultant-forces-via-the-pressure-and-shear-stress-terms-in-navier-stokes/2107 "2022-11-07T09:33:00Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\
**Post date:** [November 7, 2022, 9:33am UTC](https://community.freefem.org/t/calculating-the-resultant-forces-via-the-pressure-and-shear-stress-terms-in-navier-stokes/2107/1 "2022-11-07T09:33:00Z")

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Hello,

I would like to calculate the resultant forces that a viscous fluid exerts on a flat surface, suppose that the object is in 3D, is the total force calculated as follows?:

varFf([u1,u2,u3],[v1,v2,v3]) = int2d(Th, Allfaces)= (pressure_N.x_v1 + pressure_N.y_v2 + pressure_N.z.v3) + (tau\_xy_(v1_N.y + v2_N.x) + tau\_xz\*(v1_N.z + v3_N.x) + tau\_yz\*(v2_N.z + v3_N.y))

I took into account the pressure and shear stress terms.