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Ch 03: Motion in Two or Three Dimensions
Young & Freedman Calc - University Physics 14th Edition
Young & Freedman Calc14th EditionUniversity PhysicsISBN: 9780321973610Not the one you use?Change textbook
Chapter 3, Problem 35b

A river flows due south with a speed of 2.0 m/s. You steer a motorboat across the river; your velocity relative to the water is 4.2 m/s due east. The river is 500 m wide. How much time is required to cross the river?

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1
Identify the components of the boat's velocity. The boat's velocity relative to the water is 4.2 m/s due east, which is perpendicular to the river's flow.
Recognize that the width of the river is the distance the boat needs to travel across, which is 500 m.
Use the formula for time, \( t = \frac{d}{v} \), where \( d \) is the distance across the river (500 m) and \( v \) is the velocity of the boat relative to the water (4.2 m/s).
Substitute the values into the formula: \( t = \frac{500 \text{ m}}{4.2 \text{ m/s}} \).
Calculate the time required to cross the river using the formula from the previous step.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Relative Velocity

Relative velocity is the velocity of an object as observed from a particular frame of reference. In this problem, the boat's velocity relative to the water is given as 4.2 m/s due east, while the river flows south at 2.0 m/s. Understanding how these velocities combine is crucial for determining the boat's actual path and speed across the river.
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Vector Components

Vector components are the projections of a vector along the axes of a coordinate system. Here, the boat's velocity can be split into eastward and southward components. The eastward component is 4.2 m/s, and the southward component is due to the river's flow at 2.0 m/s. Calculating these components helps in determining the resultant velocity and the time to cross the river.
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Time Calculation

Time calculation involves determining how long it takes for an object to travel a certain distance at a given speed. In this scenario, the width of the river is 500 m, and the boat's effective velocity across the river is 4.2 m/s. Using the formula time = distance/speed, we can calculate the time required to cross the river, focusing on the eastward component of the boat's velocity.
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