Final answer:
When two long barges move in the same direction, an increase in speed for the slower barge may be observed due to the soliton effect, where it can ride the wave created by the leading barge, thus increasing its speed.
Step-by-step explanation:
When two long barges move in the same direction, the effect observed is often an increase in speed of the barge that is moving slower. This is due to a phenomenon known as the solitary wave effect or soliton effect in fluid dynamics. It can be explained by the fact that the leading barge creates a wave pattern in the water, which can act to pull the following barge forward, increasing its speed slightly. This effect is particularly noticeable on canal systems where the water is relatively calm and the movement of one barge can significantly influence the water flow pattern around another. If the barges are close enough, the second barge can ride the wave created by the first, reducing its own resistance and allowing it to move faster without additional energy. However, it's important to note that this effect can be influenced by many factors such as the distance between barges, their speed, and water conditions. Therefore, while an increase in speed is often observed, the conditions must be right for this to occur.