Engery- Falling Coffee Filter Problem

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Homework Help Overview

The discussion revolves around a physics problem involving the falling of coffee filters from a height, specifically focusing on concepts of terminal velocity and air resistance. The original poster presents a scenario where a single coffee filter and a stack of filters are dropped, prompting questions about the forces acting on them during free fall.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between gravitational force and air resistance at terminal velocity, questioning how to calculate the upward force of air resistance. There is discussion about the net force being zero at terminal velocity and the implications for the forces acting on the filters. Some participants raise questions about the time it would take for the stack of filters to reach the ground and how to approach part (c) of the problem.

Discussion Status

The discussion is ongoing, with participants providing insights into the relationship between forces and terminal velocity. Some guidance has been offered regarding the equations for air resistance and how to approach the calculations for multiple filters. However, there is still a lack of consensus on the specific calculations needed for part (c), indicating that further clarification is required.

Contextual Notes

Participants are working under the constraints of the problem as presented, with specific values for mass and fall time given. There is an implied need to consider the proportionality between air resistance and speed, as well as the effect of stacking multiple filters on the overall dynamics of the fall.

quickclick330
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You drop a single coffee filter of mass 1.4 grams from a very tall building, and it takes 47 seconds to reach the ground. In a small fraction of that time the coffee filter reached terminal speed.


(a) What was the upward force of the air resistance while the coffee filter was falling at terminal speed?
Fair = N

b) Next you drop a stack of 3 of these coffee filters. What was the upward force of the air resistance while this stack of coffee filter was falling at terminal speed?
Fair = N


(c) Again assuming again that the stack reaches terminal speed very quickly, about how long will the stack of coffee filters take to hit the ground? (Hint: Consider the relation between speed and the force of air resistance.)
Fall time is approximately ___s

Where do i need to start to begin this problem?? Thanks for the help! :-)
 
Last edited:
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What is the net force on the filter at terminal velocity?
 
zero
 
because there isn't any more change in momentum
 
quickclick330 said:
zero

exactly. So you have the upward air resistance force and downward gravitational force acting on the object... so what is the air resistance force equal to at terminal velocity?
 
ahhh...i got it. mg = Fair. Now I'm having issues with part c? would you just divide 47 by 3?
 
What is the equation of air resistance force in terms of velocity?

What is the terminal velocity in the case of the single filter... using the case of the single filter, what is the height?

What is the terminal velocity in the case of 3 filters... what is the time using the height calculated previously?
 
In the book they give us an equation for approximate air resistance?
 
quickclick330 said:
In the book they give us an equation for approximate air resistance?

what's the equation?
 
  • #10
can someone elaborate on how to get c? the previous posts don't help at all
 
  • #11
rubberduck said:
can someone elaborate on how to get c? the previous posts don't help at all

What learningphysics asked you for is reasonable. What relationship have they given you to work with, giving some proportionality between the force of air resistance and the speed of the object through the air. Is it F = k·v? F = k·(v^2)? The constant k is the "coefficient of drag", which you will be able to eliminate when you compare the terminal velocities for parts (b) and (c). You know what the resistance force equals at terminal velocity in both cases, so a ratio of the forces will give you a ratio involving the two different terminal velocities. (You don't need to know the height the filters fall through. Just call it H, since it will also cancel out in your comparison ratio.)
 

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