A cyclist, traveling 12 m/s, wears a helmet that is close to being a 36-cm-diameter sphere. The drag force acting on that helmet is approximately:
A) 1.8 N
B) 2.6 N
C) 3.2 N
D) 3.8 N

Question

Quizplus · Accepted Answer

$1.8 \mathrm {~N}$
$\text {The Reynolds number, assuming standard air, is}$
$$\operatorname { Re } = \frac { V D } { v } = \frac { 12 \times 0.36 } { 1.5 \times 10 ^ { - 5 } } = 2.86 \times 10 ^ { 5 }$$
$\text {Using the drag coefficient of $C _ { D } = 0.2$ from Fig. $8.9$, the drag force is approximately}$
$$F _ { D } = \frac { 1 } { 2 } \rho V ^ { 2 } A C _ { D } = \frac { 1 } { 2 } 1.2 \times 12 ^ { 2 } \times \pi \times 0.18 ^ { 2 } \times 0.2 = \underline { 1.76 \mathrm {~N} }$$

A Cyclist, Traveling 12 M/s, Wears a Helmet That Is