Acceleration is the rate of change of velocity over time. It can be calculated by taking the change in velocity and dividing it by the time taken. Acceleration has units of meters per second squared and represents how quickly the velocity of an object is changing. It is useful for quantifying and comparing how fast vehicles can change their speed, such as comparing how quickly a truck and sports car can accelerate from 0 to 60 miles per hour. Acceleration can be positive if an object is speeding up or negative if an object is slowing down.