A feather and a ball falling with air resistance

  • #1
BSH
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0

Homework Statement


Suppose you drop a feather and a steel ball of equal masses (0.014 kg) at the same time from a height of 2.2 m and you are told that the feather hits the floor 3.5 s after the steel ball. Estimate the value of the terminal velocity of the feather in air. Then, estimate the feather's drag coefficient.


Homework Equations


mg - cv = ma

The Attempt at a Solution


I did the calculus as I've done many times before and got that v=(mg/c)(1-e^(-ct/m)) and the terminal veocity = mg/c. The only other thing I feel I could do is assume the air resistance experienced by the ball is negligible, but I don't know how that helps me if I know so little about the feather.
 

Answers and Replies

  • #2
BSH
2
0
Follow-up: do I have to use conservation of energy?
 
  • #3
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Homework Statement


Suppose you drop a feather and a steel ball of equal masses (0.014 kg) at the same time from a height of 2.2 m and you are told that the feather hits the floor 3.5 s after the steel ball. Estimate the value of the terminal velocity of the feather in air. Then, estimate the feather's drag coefficient.


Homework Equations


mg - cv = ma

The Attempt at a Solution


I did the calculus as I've done many times before and got that v=(mg/c)(1-e^(-ct/m)) and the terminal veocity = mg/c. The only other thing I feel I could do is assume the air resistance experienced by the ball is negligible, but I don't know how that helps me if I know so little about the feather.
I think that you should be able to calculate, or at least estimate c, to get a numerical answer for the terminal velocity of the feather. You know how long the steel ball is in the air (it's safe to assume no air resistance), and you know that the feather takes 3.5 sec. longer.

I haven't worked through this problem, but that's the tack I would take.
 
  • #4
HallsofIvy
Science Advisor
Homework Helper
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967
With air resistance there is no "conservation if energy".
 

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