Velocity and acceleration, speed
Speed is the rate at which distance is covered and speed can be specified by its size alone. Velocity is a more specific quantity because it possesses direction as well as size; velocity is said to be a vector quantity. Acceleration occurs whenever the velocity of an object changes. When the velocity of an object decreases it is said to have decelerated. When an object travels at constant velocity of v m/s for a given period of time, t seconds, and a graph of velocity against time produces a straight line. Inertia is the sluggishness or resistance that a body offers to starting from rest or to a change of velocity. Velocity has both magnitude and direction and, by definition, any change of velocity is accompanied by acceleration.