Question

You can feel a force of air drag on your hand if you stretch your arm out of the open window of a speeding car. Note: Do not endanger yourself. What is the order of magnitude of this force? In your solution, state the quantities you measure or estimate and their values.

Answer 1

In $R=\frac{1}{2}D\rho A{v}^2$, we estimate that the coefficient of drag for an open palm is $D=1.00$, the density of air is $\rho =1.20kg/m^3$, the area of an open palm is $A=\left(0.100m\right)\left(0.160m\right)=1.60\times {10}^{-2}{m}^2$, and $v=29.0m/s$ ($65$ miles per hour). The resistance force is then
$R=\frac{1}{2}\left(1.00\right)\left(1.20kg/m^3\right)(1.60\times {10}^{-2}{m}^2)(29.0m/s{)}^2=8.07N$
or $R\sim {10}^{1}N$