Homework Help: Skydiver falling

1. Sep 27, 2012

Feodalherren

1. The problem statement, all variables and given/known data

A skydiver (85kg) is accelerating at 3m/s^2. What is the force of the air resistance on him?

2. Relevant equations
f=ma and other mechanics

3. The attempt at a solution
I solved for the F of the skydiver which would equal 255 N. I then used Newton's third law to conclude that the air resistance must be 255 N up. But then I started thinking... If he's accelerating, there must be a Fnet... How, then, can he accelerate if the friction from the air equals the force of gravity?

What should I have done?

2. Sep 27, 2012

SHISHKABOB

So you basically say that the air resistance is equal to and opposite to that of the force due to gravity.

If the air resistance is *equal and opposite* to the force due to gravity, would he be accelerating?

3. Sep 27, 2012

Feodalherren

That's what I mean. That's what threw me off. I have no idea how to solve for the air friction... maybe something with 10m/s^2 and 3m/s^2?

4. Sep 27, 2012

SHISHKABOB

If they're equal and opposite, then

$F_{g} = -F_{res}$

therefore

$F_{net} = F_{g} + F_{res} = 0$

can you accelerate if there is zero net force?

5. Sep 27, 2012

rcgldr

Yes.

6. Sep 27, 2012

PeterO

Newtons third law: For every force acting, there is an equal and opposite force acting

That 255N force is the net force - the vector sum of the acting forces.

You don't apply Newtons Third Law to the net force.

You should have been looking at the acting forces; which gave a net force of 255N

7. Sep 28, 2012

Feodalherren

Hmm... So... Would this be correct reasoning

85kg 10m/s^2 =800N
85kg 3m/s^2 = 255N

800-255= 545

The air friction is 545N

?

8. Sep 28, 2012

PeterO

Numerically not bad - except that 85 * 10 is 850 not 800; meaning a final answer of 595N

More appropriately.

Acting force down = weight force which is mg. If you assume a value of g = 10, that means a weight force of 850N.
Often we use the valu of 9.8 for g, which mans a weight force of 833N.

The net Force is 255N down [as you already calculated].

The other acting force is air resistance up.

Weight of 850N down, in combination with an air resistance force of 595N up is how we get a net force of 255N down, resulting in an acceleration 3 m/s^2.

9. Sep 28, 2012

Feodalherren

Woops, you're right :D.
Well, at least I figured it out. Thank you.