 An object is said to undergo acceleration if it is changing its speed or direction or both. An object traveling in a uniform circular motion at a constant speed is also said to undergo acceleration because its direction is changing. A device used for measuring acceleration is called an accelerometer. When velocity is plotted against time on a velocity vs. time graph, the acceleration is given by the slope, or the derivative of the graph. Velocity can be thought of as the integral of acceleration with respect to the time. Transverse acceleration (perpendicular to velocity), as with any acceleration which is not parallel to the direction of motion, causes change in direction. An object traveling in a straight line undergoes acceleration when its speed changes. The term "acceleration" generally refers to the change in instantaneous velocity. Given that velocity is a vector quantity, acceleration is also a vector quantity. A negative value for acceleration is commonly called deceleration. Jerk is the rate of change of an object's acceleration over time.