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Question about this problem and Walter Lewin's video

 
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Mar30-11, 05:03 PM   #1
 

Question about this problem and Walter Lewin's video

1. The problem statement, all variables and given/known data





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The problem is EXACTLY the same.

But one thing concerns me. The answer key said the force is [tex]-27\mu N \hat{i}[/tex]

From Walter Lewin's method, I got [tex]+27\mu N \hat{i}[/tex]. Help?


3. The attempt at a solution
 
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Mar31-11, 11:47 AM   #2
 
Is no one answering because no one is bothered to watch the video lol?
 
Apr5-11, 07:05 PM   #3
 
Please...
 
Apr5-11, 08:03 PM   #4
 

Question about this problem and Walter Lewin's video

Flip the paper over. Then your i-hat will point the same way his does.
 
Apr5-11, 10:35 PM   #5
 
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Quote by Antiphon View Post
Flip the paper over. Then your i-hat will point the same way his does.
@Antiphon: Nice answer!

@fp: He means left to right, not top to bottom.

LOL
 
Apr5-11, 10:48 PM   #6
 
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Seriously, Mr. Lewin clearly states that the force is toward the left.

Therefore, it's in the [tex]-\hat{i}[/tex] direction.
 
Apr6-11, 02:32 PM   #7
 
Quote by Antiphon View Post
Flip the paper over. Then your i-hat will point the same way his does.
If you flip the paper over, how can you see your work?
 
Apr6-11, 05:21 PM   #8
 
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Use thinner paper, write darker and/or use a light table.
 
Apr6-11, 05:36 PM   #9
 
...

but what if I want to do it mathematically?
 
Apr6-11, 07:37 PM   #10
 
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Quote by SammyS View Post
Seriously, Mr. Lewin clearly states that the force is toward the left.

Therefore, it's in the [tex]-\hat{i}[/tex] direction.
Did you see this?
 
Apr6-11, 11:24 PM   #11
 
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Quote by flyingpig View Post
The answer key said the force is [tex]-27\mu N \hat{i}[/tex]

From Walter Lewin's method, I got [tex]+27\mu N \hat{i}[/tex]. Help?
How do you get the force going to the right?

AM
 
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