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Ch. 7 - Applications of Trigonometry and Vectors
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161, 9780135440780, 9780136881117, 9780135440827, 9780135924891Non è quello che usi tu?Cambia libro di testo
Capitolo 8, Problema 34

Two forces of 128 lb and 253 lb act on a point. The resultant force is 320 lb. Find the angle between the forces.

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Identify the given quantities: two forces with magnitudes \(F_1 = 128\) lb and \(F_2 = 253\) lb, and their resultant force \(R = 320\) lb. We need to find the angle \(\theta\) between these two forces.
Recall the law of cosines for vectors, which relates the magnitudes of two vectors and their resultant: \(R^2 = F_1^2 + F_2^2 + 2 F_1 F_2 \cos(\theta)\)
Substitute the known values into the equation: \(320^2 = 128^2 + 253^2 + 2 \times 128 \times 253 \times \cos(\theta)\)
Rearrange the equation to isolate \(\cos(\theta)\): \(\cos(\theta) = \frac{320^2 - 128^2 - 253^2}{2 \times 128 \times 253}\)
Calculate the right-hand side value (without final numeric evaluation here), then use the inverse cosine function to find the angle: \(\theta = \cos^{-1}(\text{value from previous step})\)

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Resultant of Two Forces

The resultant force is the single force that has the same effect as the two given forces acting together. It can be found using vector addition, considering both magnitude and direction of the forces.
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Equations with Two Variables

Law of Cosines in Vector Addition

The law of cosines relates the magnitudes of two vectors and the angle between them to the magnitude of their resultant. It is expressed as R² = A² + B² - 2AB cos(θ), where θ is the angle between the forces.
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Intro to Law of Cosines

Solving for the Angle Between Forces

By rearranging the law of cosines formula, the angle between two forces can be found using θ = cos⁻¹((A² + B² - R²) / (2AB)). This allows determination of the angle when the magnitudes and resultant are known.
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Find the Angle Between Vectors