* Acceleration is a change in velocity. Velocity includes both speed and direction.
* Constant speed doesn't mean constant velocity. If an object is moving in a circle at a constant speed, its direction is constantly changing. This means its velocity is changing, and therefore it has acceleration.
Examples:
* A car driving around a roundabout: The car may be going at a constant speed, but its direction is changing, so it has acceleration. This is called centripetal acceleration.
* A ball thrown straight up: The ball slows down as it goes up (due to gravity), then speeds up as it comes down. Even though its speed changes, it still has acceleration due to the constant force of gravity.
In summary: An object can have acceleration even if its speed is constant, as long as its direction is changing.
