Average friction force on a falling apple

Click For Summary

Homework Help Overview

The problem involves calculating the average friction force acting on a falling apple, given its mass, height of fall, and final speed upon impact. The subject area includes concepts of energy conservation and forces, particularly in the context of motion through a resistive medium such as air.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of conservation of energy, questioning the relationship between potential energy, kinetic energy, and work done by friction. Some express uncertainty about the correct interpretation of force versus energy in the context of the problem.

Discussion Status

Participants are exploring different interpretations of the problem, with some suggesting that the conservation of energy approach could be valid. There is acknowledgment of the need to differentiate between energy terms and force terms, and some have provided guidance on how to relate energy loss to the work done by friction.

Contextual Notes

There is a noted confusion regarding the units of force and energy, as well as the appropriate formulas to use for calculating the friction force. Participants are questioning the assumptions made in their initial approaches and the definitions of terms used in the problem.

ObviousManiac
Messages
37
Reaction score
0

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.
 
Physics news on Phys.org
What is Ff ? Can you explicit ?
 
Quinzio said:
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.
 
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...
 
gneill said:
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!
 

Similar threads

  • · Replies 8 ·
Replies
8
Views
2K
Replies
15
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 33 ·
2
Replies
33
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 10 ·
Replies
10
Views
9K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 13 ·
Replies
13
Views
2K
Replies
2
Views
2K