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Forces and torques in Stokes flow, Show that the force acting on a control surface of any arbitrary control volume is equal to the force on a larger regularly shaped control volume enclosing the given body

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Forces and torques in Stokes flow, Show that the force acting on a control surface of any arbitrary control volume is equal to the force on a larger regularly shaped control volume enclosing the given body. In order to do this you need a control volume that is multiply connected by surfaces S1 and S2. Then apply the divergence theorem in reverse to show that the forces on S1 and S2 are the same. You should also write the Stokes equation in the following stress-divergence form: ∇ · σ˜ = 0 where σ˜ is the total stress tensor defined as −p˜I + ˜τ . Show that the torque acting on any arbitrary control volume is again given as the torque on a larger regularly shaped control volume enclosing the given body.