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Finding minimum force to pull the napkin out from under the tea cup

  • Thread starter Jpyhsics
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  • #1
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Homework Statement


There is a table, and a napkin on it. The napkin has a mass mn=71.6 g. On the napkin, there is a tea cup with mass mc=104.6 g. What is the minimum force required for someone to pull the napkin out from under the tea cup? Assume that between the table and the napkin the friction is µk= 0.484, while the coefficient of static friction between the tea cup and the napkin is µs=0.705. Express your answer in Newtons.

Homework Equations


F=ma

The Attempt at a Solution


ΣFxNapkin=F-frC-frT (where C is cup and T is table)
ΣFxCup=frN (where N is napkin)

I have also included a photo of my FBD
How can I proceed with this question? Do I have a correct FBD?
 

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Answers and Replies

  • #2
haruspex
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How can I proceed with this question? Do I have a correct FBD?
Your FBD looks fine.
What about the cup's acceleration?
 
  • #3
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Your FBD looks fine.
What about the cup's acceleration?
Would the acceleration of the cup be equal to the acceleration of the napkin?
 
  • #4
haruspex
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Would the acceleration of the cup be equal to the acceleration of the napkin?
Yes, or only very slightly less. So what equation can you write?
 
  • #5
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so would the cup acceleration be:
ackg ?
Which subsequently would be the acceleration of the napkin?

So would my solution in the picture make sense?
 

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  • #6
haruspex
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so would the cup acceleration be:
ackg ?
μk is the kinetic friction between the napkin and the table.
 
  • #7
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Okay thanks!
 
  • #8
haruspex
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Okay thanks!
Not sure how to interpret that response. I am telling you that your equation in post #5 makes no sense... and wondering if it was a typo.

Edit: I have marked the thread unsolved.
 
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