Consider a car moving with a **constant, rightward (+) velocity** – say of +10 m/s. As learned in an earlier lesson, a car moving with a constant velocity is a car with zero acceleration.

If the velocity-time data for such a car were graphed, then the resulting graph would look like the graph at the right. Note that a motion described as a constant, positive velocity results in a line of zero slope (a horizontal line has zero slope) when plotted as a velocity-time graph. Furthermore, only positive velocity values are plotted, corresponding to a motion with positive velocity.

Now consider a car moving with a **rightward (+), changing velocity** – that is, a car that is moving rightward but speeding up or *accelerating*. Since the car is moving in the positive direction and speeding up, the car is said to have a *positive* acceleration.

If the velocity-time data for such a car were graphed, then the resulting graph would look like the graph at the right. Note that a motion described as a changing, positive velocity results in a sloped line when plotted as a velocity-time graph. The slope of the line is positive, corresponding to the positive acceleration. Furthermore, only positive velocity values are plotted, corresponding to a motion with positive velocity.

The velocity vs. time graphs for the two types of motion – constant velocity and changing velocity (acceleration) – can be summarized as follows.

Positive VelocityZero Acceleration | Positive VelocityPositive Acceleration |

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