Answer
144N
Work Step by Step
When coasting down, there are two forces acting on the bicyclist, the force of gravity and the force of air resistance. The force of gravity is in the negative direction and the force of air resistance is in the positive direction so that net force is 0.
$\sum F_N=-F_G+F_{AR}=0$
$F_G=F_{AR}$
$F_{AR}=mg\sin\theta$
$F_{AR}=(65kg)(9.8\frac{m}{s^2})\sin(6.5^o)=72.1N$
When climbing up the hill, the force of gravity is still downward but the force of air resistance is also downward. The magnitude of the net force is the sum of the two forces.
$\sum F_N=F_G+F_{AR}=2F_{AR}$
$\sum F_N=2(72.1N)=144N$
