Final answer:
The wave speed decreases as they move up the rope because the tension in the rope decreases the higher up the wave travels.
Step-by-step explanation:
When waves are started from the bottom of a rope that is suspended from the ceiling and travel upwards, their speed changes due to the varying tension along the length of the rope. As the wave moves up the rope, the portion of the rope above the wave has less mass to support than the portion below. Therefore, the tension in the rope decreases as the wave travels upwards, which, according to the relationship between tension and wave speed, leads to a slower wave speed. This is because the wave speed on a string is directly proportional to the square root of the tension and inversely proportional to the square root of the linear mass density (wave speed = √(T/μ), where T is tension and μ is mass per length).