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Ch. 7 - Logarithmic, Exponential Functions, and Hyperbolic Functions
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 7, Problema 7.3.95a

Falling body When an object falling from rest encounters air resistance proportional to the square of its velocity, the distance it falls (in meters) after t seconds is given by d(t) = (m/k) ln (cosh (√(kg/m) t)), where m is the mass of the object in kilograms, g = 9.8 m/s² is the acceleration due to gravity, and k is a physical constant.


a. A BASE jumper (m = 75 kg) leaps from a tall cliff and performs a ten-second delay (she free-falls for 10 s and then opens her chute). How far does she fall in 10 s? Assume k = 0.2.

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Identify the given function for the distance fallen: \(d(t) = \frac{m}{k} \ln \left( \cosh \left( \sqrt{\frac{kg}{m}} \, t \right) \right)\), where \(m\) is mass, \(k\) is a constant, \(g = 9.8\) m/s², and \(t\) is time in seconds.
Substitute the known values into the formula: \(m = 75\), \(k = 0.2\), \(g = 9.8\), and \(t = 10\) seconds.
Calculate the term inside the square root: \(\sqrt{\frac{kg}{m}} = \sqrt{\frac{0.2 \times 9.8}{75}}\).
Evaluate the argument of the hyperbolic cosine function: \(\sqrt{\frac{kg}{m}} \times t = \left( \text{value from previous step} \right) \times 10\).
Compute the distance fallen by plugging the value into the formula: \(d(10) = \frac{75}{0.2} \times \ln \left( \cosh \left( \text{value from previous step} \right) \right)\).

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Modeling Air Resistance in Free Fall

Air resistance affects falling objects by opposing motion, often modeled as proportional to velocity squared for high speeds. This nonlinear drag force changes the acceleration and velocity over time, making the motion differ from simple free fall. Understanding this helps interpret the given distance formula involving hyperbolic functions.
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Exponential Growth & Decay

Hyperbolic Functions and Their Properties

Hyperbolic functions like cosh(x) and sinh(x) arise in solutions to differential equations involving quadratic velocity terms. The function cosh(x) = (e^x + e^{-x})/2 grows exponentially and appears in the distance formula, reflecting the balance between gravity and air resistance in the falling motion.
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Percorso guidato
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Properties of Functions

Applying Given Formulas with Physical Constants

To find the distance fallen, substitute the known values (mass m, gravity g, constant k, and time t) into the formula d(t) = (m/k) ln(cosh(√(kg/m) t)). This requires careful calculation of the square root term and the natural logarithm, ensuring units are consistent and the physical meaning is preserved.
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Example 5: Packaging Design
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Evaluating hyperbolic functions Use a calculator to evaluate each expression or state that the value does not exist. Report answers accurate to four decimal places to the right of the decimal point.

a. coth 4

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Terminal velocity Refer to Exercises 95 and 96.


a. Compute a jumper’s terminal velocity, which is defined as lim t → ∞ v(t) = lim t → ∞ √(mg/k) tanh (√(kg/m) t).

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Evaluating hyperbolic functions Evaluate each expression without using a calculator or state that the value does not exist. Simplify answers as much as possible.


a. cosh 0

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