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The frame time derivative, valid for a fixed point in either the material or the spatial frame. This derivative is denoted ut in some physics interfaces. It is defined either as
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The mesh time derivative, which is taken for a fixed point in the mesh:
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While the variable t represents the current value of the time in all frames, differentiation must be performed with respect to the symbol TIME only. The operator expression d(<expr>,TIME) is always allowed and evaluates to the mesh time derivative of <expr>. Because of its ambiguous meaning, the expression d(<expr>,t) is illegal if <expr> contains any variables that are functions of space.
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