# Homework Help: A Problem Involving Newton's Second Law

1. Sep 17, 2011

### Manni

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.

Hey guys, I need a help on how to approach this problem. Initially, I assumed Fr = Ff = umg. But the problem states that Fr is proportional to velocity. Help?

2. Sep 17, 2011

### vela

Staff Emeritus
Newton's 2nd law is
$$\sum_i \mathbf{F}_i = m\mathbf{a}$$What do you get when you apply it to this problem?

3. Sep 17, 2011

### Xyius

Proportional meaning, the upward force equals the velocity, times a proportionally constant. Hence the drag force is Fr=bv, where b is a real number and v is velocity.

4. Sep 17, 2011

### Manni

Thanks Xyius! Using your definition of drag force, I was able to work it out!

5. Sep 17, 2011