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Continuum mechanics/Relations between surface and volume integrals

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Surface-volume integral relation 1

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Let be a body and let be its surface. Let be the normal to the surface. Let be a vector field on and let be a second-order tensor field on . Show that

Proof:

Recall the relation

Integrating over the volume, we have

Since and are constant, we have

From the divergence theorem,

we get

Using the relation

we get

Since and are constant, we have

Therefore,

Since and are arbitrary, we have


Surface-volume integral relation 2

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Let be a body and let be its surface. Let be the normal to the surface. Let be a vector field on . Show that

Proof:

Recall that

where is any second-order tensor field on . Let us assume that . Then we have

Now,

where is any second-order tensor. Therefore,

Rearranging,