Ground speed vs Air speed
There is formula for on wiki article "Bicycle performance" known as
P = g * m * V_g * (K_1 + s) + K_2 * (V_a)^2 * V_g
P - Power in watts
g - gravity
m - mass of the rider plus bike
s - grade of surface
V_g - ground speed m/s
V_a - air speed
K_1 - a constant which represents frictional losses
K_2 - a constant representing aerodynamic drag
My question is why do we have both a V_g (ground speed) and V_a (Air speed)? Wouldn't these two always be the same?