No, acceleration doesn't only speed up an object. It's actually a more general concept that describes any change in an object's velocity.

Here's why:

* Velocity is a combination of speed and direction.

* Acceleration is the rate of change of velocity.

So, acceleration can cause:

* Increasing speed: This is what we usually think of when we hear "acceleration". A car speeding up is an example.

* Decreasing speed: This is called deceleration or retardation. A car slowing down to a stop is an example.

* Changing direction: Even if the speed remains constant, a change in direction means the velocity has changed, and therefore acceleration is present. A car turning a corner is an example.

In summary: Acceleration is any change in velocity, not just speeding up.