Running on a treadmill is slightly easier than running outside because there is

Question

Running on a treadmill is slightly easier than running outside because there is no drag force to work against. Suppose a 60 kg runner completes a 5.0 km race in 18minutes.

Determine the drag force on the runner during the race. Suppose that the cross section area of the runner is 0.72 m2 and the density of air is 1.2 kg/m3

What is this force as a fraction of the runner's weight?

I have figured out the first part but am having difficulty getting the correct answer for D/w ( drag force as a fraction of runner's weight.)

Explanation / Answer

v = s/t = 5000/(18*60) = 4.63 m/s

Fd = 0.5*rho*v^2*Cd*A

Fd = 0.5*1.2*4.63^2*1.3*0.72

Fd = 12.03 N

fraction = Fd/mg = 0.02

drag force as a fraction of runner's weight = drag force/runner's weight = drag force/(60*9.8) N

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