Acceleration is the rate of change in speed. If the acceleration is uniform the it changes at a constant rate, and it can be calculated by dividing the the difference between its final and initial speeds over a time interval by the elapsed time:
`a = (Delta v)/(Delta _t)`
`a = (60.0m/s - 35.0 m/s)/(15.0 s) = 1.67 m/s^2`
Acceleration has units of meters per second^2 or meters per second per second.
Acceleration is a vector quantity, as is velocity. As such it should have a direction specified. Acceleration can also be negative, as is the case when an object's velocity or speed is decreasing.