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Ch 09: Work and Kinetic Energy
Chapter 9, Problem 9

When you ride a bicycle at constant speed, nearly all the energy you expend goes into the work you do against the drag force of the air. Model a cyclist as having cross-section area 0.45 m² and, because the human body is not aerodynamically shaped, a drag coefficient of 0.90 . Use 1.2 kg/m³ as the density of air at room temperature. (c) The food calorie is equivalent to 4190 J. How many calories does the cyclist burn if he rides over level ground at 7.3 m/s for 1 h?

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Key Concepts

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

Drag Force

The drag force is the resistance experienced by an object moving through a fluid, such as air. It is influenced by the object's speed, cross-sectional area, and shape, as well as the fluid's density. The drag force can be calculated using the formula F_d = 0.5 * C_d * A * ρ * v², where C_d is the drag coefficient, A is the cross-sectional area, ρ is the fluid density, and v is the velocity.
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Work and Energy

Work is defined as the energy transferred when a force is applied to an object over a distance. In the context of cycling, the work done against drag force translates into energy expenditure. The relationship between work, force, and distance can be expressed as W = F * d, where W is work, F is the force applied, and d is the distance traveled. This energy expenditure is often measured in joules or calories.
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Caloric Expenditure

Caloric expenditure refers to the amount of energy used by the body during physical activity, often measured in calories. One food calorie is equivalent to 4190 joules, and understanding this conversion is essential for calculating how many calories a cyclist burns over a given time and speed. By determining the work done against drag force, one can estimate the total energy expended and convert it into calories.
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