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Two canoeists in identical canoes exert the same effort paddling and hence maintain the same speed relaLive to the water. One paddles directly upstream (and moves upstream), whereas the other paddles directly downstream. With downstream as the positive direction, an observer on shore determines the velocities of the two canoes to be \(-1.2 \mathrm{~m} / \mathrm{s}\) and \(+2.9 \mathrm{~m} / \mathrm{s}\), respectively. (a) What is the speed of the water relative to the shore? (b) What is the speed of each canoe relative to the water?

Short Answer

Expert verified
The speed of the water relative to the shore is \(0.85 m/s\) and the speed of each canoe relative to the water is \(2.05 m/s\).

Step by step solution

01

Determine the velocity of water

The velocity of water relative to the shore can be obtained by finding the average velocity of the two canoes. Since the canoes exert the same effort, the speed of the water should result in the average velocity of the two canoes. Therefore, it can be calculated as follows: \[ v_w = \frac{v_{upstream} + v_{downstream}}{2} = \frac{-1.2 m/s + 2.9 m/s}{2} \]
02

Calculate the velocity of the canoe relative to the water

Since the canoes exert the same effort, their speeds relative to the water are the same. Let's denote the speed of the canoe relative to the water as \( v_c \). We know that the speed of an object, in this case, a canoe, relative to the shore equals to the difference between its velocity and the velocity of the water : \( v_{downstream} - v_w = v_c \) and \( v_w - v_{upstream} = v_c \). It doesn't matter which one to use as both equations should give us the same result. Let's calculate the speed as: \[ v_c = v_{downstream} - v_w \] and \[ v_c = v_w - v_{upstream} \]
03

Calculation

Now plug in the values we have: From Step 1, we found that the speed of the water \( v_w = 0.85 m/s \). Now we can finalize the second step by calculating speed of a canoe: \( v_c = 2.9 m/s - 0.85 m/s \) and also \( v_c = 0.85 m/s - (-1.2 m/s) \)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Velocity of Water Relative to Shore
Understanding how the velocity of water compares to a stationary reference point, like the shore, is crucial in relative motion problems. Imagine you're standing on the riverbank, and you notice leaves floating down the stream. Without knowing it, you're observing the velocity of water relative to the shore.

Now, let's dive into our exercise example. We have two canoes moving in opposite directions - one upstream and the other downstream. By averaging their velocities, we can determine the water's speed. Think of it as if you’re put in the middle of the moving water and are trying to even out the effects of the current in both directions. This average gives us the velocity of the water relative to the shore, providing us with a baseline to understand how fast the current itself moves when unaffected by other forces.
Speed of Canoe Relative to Water
Imagine you're a canoeist paddling in a river; your efforts against the water dictate how fast you go. Now, envision your motion in relation to the flowing water beneath you. That's what we mean by the 'speed of the canoe relative to the water.'

Our textbook scenario assumes identical canoes and efforts, leading to the same relative speed for each canoe. We determine the individual speed of the canoe in still water by considering the water's speed as a conveyor belt moving beneath the canoe. If you paddle downstream, the current aids you; paddling upstream, it resists you. By comparing the canoe's velocity with and against the current, we can extract the influence of the water's movement and reveal the canoe's own paddling speed.
Relative Motion Analysis
Relative motion analysis is the study of how different objects move in relation to one another. In physics, it's not just about how fast something is moving, but also how that movement is observed from different reference points.

In the context of our canoe problem, we're considering two reference points: the moving water and the stationary shore. By examining how the canoes behave in each frame of reference (with the shore as an observer or while in the water), we can unravel the puzzle of each canoe's actual speed, as well as the speed of the river's flow. This kind of analysis is fundamental in understanding problems involving vehicles, aircraft, ships, and even celestial bodies in space.
Average Velocity Calculation
Calculating the average velocity serves as a vital concept in this scenario. The average velocity isn't just about a simple mean value; it’s the balance point of a system in relative motion.

To find the water's speed relative to the shore, we calculated the average of the canoes' velocities headed in opposite directions. By doing so, we're essentially nullifying the push and pull effect of the river's flow and obtaining a neutral perspective of the water's movement. Our solution emphasizes that average velocity is not only about finding the middle ground in terms of speed but also about interpreting physical phenomena in a broader context where multiple movements intersect.

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