Here's why:
* Acceleration is a vector: It has both magnitude (how much) and direction.
* Speed is a scalar: It only has magnitude (how fast).
Examples:
* Driving in a circle at constant speed: You are constantly accelerating because your direction is changing even if your speed remains the same.
* Swinging a ball on a string: The ball changes direction continuously, so it accelerates even though its speed might be constant.
In other words: Acceleration measures any change in velocity, and velocity is a combination of speed and direction. So, even if your speed is constant, if your direction is changing, you are accelerating.
