* Acceleration is the rate of change of *velocity*. Velocity includes both speed and direction.
* Speed is only the magnitude (how fast) of the velocity.
Here are some examples:
* An object moving in a circle at constant speed: The object's speed remains the same, but its direction is constantly changing. This means its velocity is changing, and therefore it is accelerating.
* An object moving in a straight line at constant speed: The object's speed and direction remain constant. This means its velocity is not changing, and therefore it is not accelerating.
In summary:
* A change in speed always indicates a change in velocity.
* A change in velocity (and therefore acceleration) can be caused by a change in speed, a change in direction, or both.
