7.2
Arc Length of a Curve: Problem Solving
A high-voltage power line spans a 40-meter horizontal distance between two transmission towers, resulting in a 10-meter vertical sag due to the effect…
A high-voltage power line hanging between two transmission towers spans a 40-meter gap with a significant 10-meter vertical sag due to thermal expansion.
The objective is to find the exact length of the power line required to connect these points safely.
Engineers model this using a catenary, the natural curve of a hanging cable described mathematically by the hyperbolic cosine function.
This curve is controlled by a parameter a, which relates the cable’s weight per meter to the horizontal tension. For this specific installation, the value of parameter a is assumed to be 20 meters.
To find the exact length, the calculation uses the arc length function. Here, the derivative of the catenary gives the hyperbolic sine.
A standard identity involving hyperbolic functions simplifies the expression further by removing the square root.
The integral of the resulting hyperbolic cosine is simply the hyperbolic sine. Evaluating the hyperbolic sine function at the limits from negative twenty to positive twenty gives a total length of approximately 47 meters.
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Q1: Why is a catenary curve used to model a hanging power line instead of a parabola?
A catenary accurately describes a uniform, flexible cable hanging under its own weight, unlike a parabola which is merely an approximation. The catenary is defined by the hyperbolic cosine function and precisely represents how gravity and the cable's weight distribution affect its shape. This mathematical model ensures engineers can calculate exact cable lengths needed for safe installation between transmission towers.
Q2: What does the parameter 'a' represent in the catenary equation for a power line?
The parameter a represents the ratio between the horizontal tension in the cable and its weight per unit length. In the power line example, a equals 20 meters. This parameter controls the catenary's shape and determines how much the cable sags under its own weight and thermal expansion effects.
Q3: How does the arc length function calculate the exact length of a suspended cable?
The arc length function integrates the derivative of the catenary curve, which yields the hyperbolic sine function. A standard hyperbolic identity simplifies the expression by eliminating the square root. Integrating the resulting hyperbolic cosine and evaluating at the tower positions gives the total cable length needed for the installation.
Q4: Why is the origin placed at the lowest point of the catenary curve in this power line problem?
Placing the origin at the lowest point creates a symmetric coordinate system where the two transmission towers are positioned equidistantly on either side. For this 40-meter span with parameter a of 20 meters, the towers are located 20 meters to the left and right of the curve's minimum. This symmetry simplifies the integration limits and calculations.
Q5: What is the relationship between hyperbolic sine and hyperbolic cosine in arc length calculations?
The derivative of the catenary's hyperbolic cosine function produces the hyperbolic sine. When computing arc length, a standard hyperbolic identity involving these functions eliminates the square root from the integrand. The integral of the simplified hyperbolic cosine expression yields the hyperbolic sine, which is then evaluated at the integration limits.
Q6: How much cable length is required for a 40-meter power line span with 10-meter sag?
For a 40-meter horizontal span between towers with a 10-meter vertical sag, the catenary model with parameter a of 20 meters yields approximately 47 meters of cable length. This additional 7 meters accounts for the curve's natural shape under gravity and thermal expansion, ensuring the cable hangs safely without excessive tension.
Q7: What role do hyperbolic functions play in modeling real-world engineering problems?
Hyperbolic functions like hyperbolic cosine and sine naturally describe physical phenomena involving hanging cables and structures under their own weight. These functions appear in applications of integration across engineering and physics. Their mathematical properties, including standard identities, enable engineers to solve complex real-world problems such as determining exact cable lengths for power line installations.