Analyzing Departures from Linearity Practice Test

A psychologist records number of practice trials ($x$) and reaction time ($y$, in milliseconds) for a task. The scatterplot shows reaction time decreasing quickly at first and then approaching a minimum, forming a curve. Which feature suggests a linear model is not appropriate?

A physics student measured the stopping distance (y) of a toy car versus its initial speed (x). The scatterplot shows stopping distance increases slowly at low speeds but much more rapidly at higher speeds. Which feature suggests that a linear model may not be appropriate?