Average friction force on a falling apple

[ObviousManiac]

Homework Statement

"A 0.15 kg apple falls from a branch 20 m above the ground. If it hits the ground with a speed of 17 m/s, what was the average friction force (due to air resistance) exerted on the apple?"

Homework Equations

At first, I thought I would use conservation of energy, PE = KE + Ff (where Ff is force due to friction) but I think there's something wrong with that.

The Attempt at a Solution

Using conservation of energy I found Ff to be 7.725 (Joules? I think this should be in Newtons...), but I would expect it to be significantly lower.

 

[Quinzio]

What is Ff ? Can you explicit ?

 

[ObviousManiac]

What is Ff ? Can you explicit ?

Force due to friction = µN where µ = coefficient of friction and N = Normal Force

I'm skeptical as to whether I approached the problem with the appropriate formula.

 

[gneill]

Your conservation of energy approach could work. Note that you will be dealing with energy terms, not force terms (at least to begin with).

The energy lost to friction on the way down will be the difference between the initial potential energy (due to gravity and height) and the final kinetic energy (due to the achieved velocity).

If you call that energy difference the work done by friction, then that work would be due to some average force due to the friction applied over the distance of the fall...

 

[ObviousManiac]

If you call that energy difference the work done by friction, then that work would be due to some average force due to the friction applied over the distance of the fall...

Ahhh of course! Thanks, for some reason my mind wasn't working right with this problem.

So with that in mind...

PE = KE + Wf (where Wf is work done by Friction)

Difference in Energy comes out to be 7.725
Divided by Distance (20m...)
Friction Force comes out to be .386 N

Much more realistic. Thanks for the help!