Which of the following best defines acceleration?

Acceleration is defined as a change in speed over time. This concept is crucial in understanding how an object’s velocity changes in relation to time. When an object accelerates, it is either increasing or decreasing its speed, indicating that the rate of change of the object's velocity is not constant. This definition encompasses both positive acceleration (speeding up) and negative acceleration (slowing down, also referred to as deceleration).

In contrast, a change in direction only describes a scenario that could involve acceleration if there is a change in speed, but it does not encompass all aspects of acceleration. A constant speed with no change indicates that the object is not accelerating at all; instead, it is maintaining a uniform velocity. Motion in a straight line represents a specific type of movement, which can involve acceleration when the speed changes, but does not inherently define acceleration itself. Therefore, the best definition of acceleration is indeed the change in speed over time.