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 [itex]F_r[/itex] is proportional to velocity v, we can write

[tex]F_r=kv[/tex] where k is constant of proportionality.

[tex]\Rightarrow mg-kv=ma[/tex]

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

[tex]\therefore a = g-\frac{kv}{m}[/tex]

 

[Chosen-wun]

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

[tex]F_r=kv[/tex] where k is constant of proportionality.

[tex]\Rightarrow mg-kv=ma[/tex]

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

[tex]\therefore a = g-\frac{kv}{m}[/tex]

That makes a lot more sense, thank you.