Acceleration can only occur when the speed of an object changes.

False.

In fact, acceleration occurs when there is a change in velocity of an object. Keep in mind that the speed of an object is the magnitude of the velocity: therefore, acceleration can also occur when there is a change in the direction of the velocity, while the speed remains constant. An example of this is the uniform circular motion, where the centripetal acceleration is due to the fact that the velocity constantly changes its direction, but its speed remains constant.