Simplify the following text for a Grade 7 native English-speaking person.
Original: In physics , a change in the direction of velocity also is an acceleration : for rotary motion , the change in direction of velocity results in centripetal ( toward the center ) acceleration ; whereas the rate of change of speed is a tangential acceleration .
Simplified: Acceleration is the change of velocity divided by the change of time . Acceleration is a vector , and therefore includes both a size and a direction .