Problem using Newtons second law

[Chosen-wun]

Once her chute opens, a sky-diver of mass m is acted upon by a downward force Fg due to gravity, and an upward force Fr due to air resistance. If Fg = mg, where g is gravitational acceleration, and Fr is proportional to velocity v, use Newton’s Second Law of Motion to write acceleration a as a function of velocity v.

I don't know if I'm on the right track
Fnet = ma
fnet = Fg+ Fr
Fg = mg

Mg + Fr = ma

 

[issacnewton]

yes on right track except the signs. Now since F_r is proportional to velocity v, we can write

F_r=kv where k is constant of proportionality.

\Rightarrow mg-kv=ma

the negative sign is there since the air drag opposes the downward motion,

\therefore a = g-\frac{kv}{m}

 

[Chosen-wun]

yes on right track except the signs. Now since F_r is proportional to velocity v, we can write

F_r=kv where k is constant of proportionality.

\Rightarrow mg-kv=ma

the negative sign is there since the air drag opposes the downward motion,

\therefore a = g-\frac{kv}{m}

That makes a lot more sense, thank you.