June 14, 2015
June 14, 2015
June 17, 2015
Engineering Physics & Physics
26.1273.1 - 26.1273.31
Projectile Motion with Aerodynamic Drag: The Cubic LawA classic problem covered in engineering mechanics and physics courses is the determination ofthe trajectory for projectile motion. The version of the problem traditionally introduced does notinvolve any resistance effect due to the medium through which the projectile travels. However,in terrestrial applications, the atmosphere present inherently produces an aerodynamic drag forcethat noticeably alters the trajectory in comparison to the trajectory that would occur in a vacuum.The primary reason for neglecting the drag force is the mathematical complications that arise inthe governing equations of motion if a drag force effect is included.In realistic applications, it can be shown that the drag force on the projectile can be appropriatelymodeled as being proportional to projectile-speed squared. An exact solution to the equations ofmotion for this model can be obtained, but it involves quadratures which cannot be analyticallyevaluated in terms of standard functions. An approximate solution that is remarkably accurate ispresented in this article. This solution is based upon the so-called cubic law, which is motivatedby certain properties of the exact solution. It involves a third-degree polynomial equation whichis utilized to generate the projectile trajectory. Because of the relative simplicity of this familiaralgebraic equation, estimates can be readily obtained for other quantities of interest, such as themaximum projectile range and optimal projection angle. Although they are approximate, theseestimates provide insight to students about the essential differences in the results for the realisticand idealized versions of this classic problem.An intriguing characteristic of the approximate solution for projectile motion with aerodynamicdrag included is that it has the same form as the exact solution for projectile motion in a vacuum(a well-known second-degree polynomial equation, whose graph is an inverted parabola), exceptfor the addition of a single cubic term.The author has utilized the approximate solution to create an engaging programming exercise forstudents enrolled in an engineering computation course. In this course, the students are tasked toconstruct an EXCEL® spreadsheet with charts (graphs) to display the trajectories for a projectileunder a variety of conditions. It is anticipated that this spreadsheet will be briefly demonstratedduring the formal presentation of this article.
Hayen, J. C. (2015, June), Projectile Motion with Aerodynamic Drag: The Cubic Law Paper presented at 2015 ASEE Annual Conference & Exposition, Seattle, Washington. 10.18260/p.24610
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