Solving Physics Gravity Problem: 80kg Object Falling

[SwAnK]

Alrite, I am in gr. 11 academic physics, and have a pretty good understaning of it so far. Recently we were given an assignment and i need some clarifcation for this one question.

The question:
a 80kg object is free falling and when it reaches a velocity of 20m/s (down) air resistence acts on it to make it slow down at 0.2m/s/s (up)
Find the air resistence.
Also find its final velocity and how far it falls after 3 seconds of the force acting. How long after it was released from rest did the air resistence begin to react.

What i need clatriy on is air resistence. Would you use Fnet=gm, or Fnet=ga to find it and do u have to use the initial velocity somehow. Or is initial velocity only there for the other parts of the question? i think my answer for the air resistence was around 1.96N.

 

[tony873004]

I would guess that it wants the force from air resistance.
f=ma
f = 80 kg * 0.2 m/s/s = 16N

How long after it was released did the air resistance begin to react. It started after it reached 20 m/s, and it accelerated at 9.8m/s/s to get to 20 m/s.

It's final velocity after 3 seconds:
It spent 2 seconds accelerating at 9.8 m/s/s
and
1 second accelerating at 9.8-0.2 m/s/s

Your distance will be your average velocity * 3 seconds

 

[HallsofIvy]

"What i need clatriy on is air resistence. Would you use Fnet=gm, or Fnet=ga to find it and do u have to use the initial velocity somehow. Or is initial velocity only there for the other parts of the question? i think my answer for the air resistence was around 1.96N."

net force is neither gm not ga (ga makes no sense at all! g is itself an acceleration).

The question seems to me to be ambiguous. Does "air resistence acts on it to make it slow down at 0.2m/s/s (up)" refer to the net acceleration or only the acceleration due to air resistance, ignoring gravity? I assumed net acceleration in what I did below. tony873004 assumed only air resistance and, upon reflection, I think he is right.

The net force is -mg+ F<sub>air[/b] where "Fair" is the force of air resistance. You know (I hope) that g is 9.8 m/s² so -80(9.8)+Fair= (80)0.2 m/sec². Fair= 80(0.2+ 9.8)= 80(10)= 800 Newtons (upward). I have no idea how you got 1.98!
(The 16 Newtons tony873004 got was the net force- including gravity.)

Apparently, according to this problem, there was no air resistance until the downward speed was 20 m/s. How long does it take something, accelerating at 9.8 m/s² to reach 20 m/s?</sub>

 

[gschjetne]

I'd skip the over-complication and use these formulas:

$$F_d=C_d \frac{1}{2} \rho v^2$$

This would be the force of drag formula. I know that at terminal velocity $F_d=mg$, Therefore

$$C_d= \frac{mg}{\frac{1}{2} \rho v^2}$$

Please correct me if I'm wrong, though, as the assignment really didn't say much about what kind of figure they were looking for, wether it was force or coefficient...

 

[SwAnK]

Thanx for all the help guys. I did some stupid mistakes when doing the question. I tired again, and got pretty much the same as tony did, so that's good.
thanx again