# Other Rates of Change (Volume and Area)

For completeness, we’ll quickly derive the rate of volume change and the rate of area change:

Volume:
If , where , , are the lengths of the sides of a “box” that is oriented in the principal directions of , then:

(1)

Area:
Consider the following time derivative:

where is an area (as opposed to , , , which are lengths).

is known from eq. 2 in Section 2: Volume and Area Change. So, we can take the time derivative. We can also re-write in terms of .

(2)

The derivation was skipped here, but can be found in [Holzapfel].

We’ll come back to rates again when we get to rate-form constitutive relationships.