Review. The instantaneous position of an object is specified by its position vector leading from a fixed origin to the location of the object, modeled as a particle. Suppose for a certain object the position vector is a function of time given by $\overrightarrow{\mathbf{r}}=4 \hat{\mathbf{i}}+3 \hat{\mathbf{j}}-2 t \hat{\mathbf{k}},$ where $\overrightarrow{\mathbf{r}}$ is in meters and $t$ is in seconds. (a) Evaluate $d \overrightarrow{\mathbf{r}} / d t$.
(b) What physical quantity does $d \overrightarrow{\mathbf{r}} / d t$ represent about the object?