Final answer:
A particle with constant speed can be accelerating if its direction changes. A particle with constant velocity has zero acceleration.
Step-by-step explanation:
A particle with constant speed can be accelerating. Acceleration refers to a change in velocity, which includes both speed and direction. A particle with constant speed can be accelerating if its direction changes, even though its speed remains the same. For example, an object moving in a circular path at a constant speed is constantly changing direction and therefore experiencing acceleration.
If a particle has constant velocity, it means that both its speed and direction are constant. In this case, the acceleration would be zero since there is no change in velocity. If an object's velocity is constant, it means it is neither speeding up nor slowing down, and there is no acceleration.