Final answer:
Canoeing upstream near the shore reduces paddling effort because the current is slower, while canoeing downstream in the middle takes advantage of the faster current. Kayaking involves the classical addition of velocities—where the kayak's total velocity is the sum of its velocity and the water's velocity relative to the riverbank.
Step-by-step explanation:
When paddling a canoe upstream, it is recommended to travel as near to the shore as possible because the water velocity against the shore tends to be lower, making it easier to paddle against the current. In contrast, when canoeing downstream, staying near the middle of the river can be beneficial since the current is stronger, potentially giving the canoe additional speed without requiring excessive paddling effort. This strategic positioning relates to classical physics, where the velocity of the boat is influenced by the velocity of the water in different parts of the river.
The motion of a kayak in a river serves as an example of the classical addition of velocities where velocities add as vectors. This concept explains that the total velocity of the kayak is the vector sum of its velocity relative to the water and the water's velocity relative to the riverbank. For instance, if a kayak maintains a stationary position in a fast-moving river, the paddler must exert effort to offset the downstream pull of the current. This is because the kayak experiences a combined velocity effect from both its efforts against the water and the water's flow along the riverbank.