Karyn is running in the Boston Marathon with an average speed of 3.35 m/s. The d

Question

Karyn is running in the Boston Marathon with an average speed of 3.35 m/s. The density of the air is 1.2 kg/m3 , Karyn’s cross-sectional area is 0.50 m2 , and the coefficient of drag is 1.1. As Karyn approaches Heartbreak Hill, the head wind picks up to 2 m/s. a) Calculate drag force that Karyn experiences. b) As she faces the headwind, Karyn instinctively crouches to reduce her cross-sectional area to 0.45 m2 . What is the new drag force that Karyn experiences? c) Can the flow of the air as it passes Karyn be described as laminar flow or turbulent flow? Why?

Explanation / Answer

F = 1/2*ro*u^2*C*A

F is the drag force, which is by definition the force component in the direction of the flow velocity,[1]ro is the mass density of the fluid, [2]u is the flow velocity relative to the object,A is the reference area, andC is the drag coefficient

u = 3.35+2 = 5.35 m/s

a) Thus, F = 1.2 x 5.35 x 5.35 x 1.1 x 0.5 /2 = 9.44 N

b) F = 1.2 x 5.35 x 5.35 x 1.1 x 0.45 /2 = 8.5 N

c) Flow of air is laminar as the wind velocity is less.

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