Wich is the rate at wich velocity changes

Velocity: The rate of change of displacement of an object (displacement over elapsed time) is velocity. Velocity is a vector since it has both magnitude (called speed) and direction. For example, if you drive 10 miles North in 0.25 hours (15 minutes), your velocity is 10 miles/0.25 hours = 40 mph in the northerly direction. For further explanation of vectors click here.

Acceleration: The rate of change of velocity is acceleration. Like velocity, acceleration is a vector and has both magnitude and direction. For example, a car in straight-line motion is said to have forward (positive) acceleration if it is speeding up and rearward (negative) acceleration if it is slowing down. We sometimes refer to negative acceleration as deceleration.