# Homework Help: Problem using newtons second law

1. Sep 19, 2011

### 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

2. Sep 19, 2011

### 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,

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

3. Sep 19, 2011

### Chosen-wun

That makes a lot more sense, thank you.