Newtons 2nd Law

$F=M{x}^{{}^{\prime \prime }}$ or $M=I{\theta }^{{}^{\prime \prime }}$

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In Fluids

$\rho a=F_{pressure}+F_{friction}+F_{gravity}$

which are the forces on a fluid parcel. Where these parcels deform according to General Planar Motion, and the linear types of motion defined in Position Analysis.