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

The problem is EXACTLY the same.

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

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

3. The attempt at a solution

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 Is no one answering because no one is bothered to watch the video lol?

Flip the paper over. Then your i-hat will point the same way his does.

Mentor
 Quote by Antiphon Flip the paper over. Then your i-hat will point the same way his does.

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

LOL

 Mentor Seriously, Mr. Lewin clearly states that the force is toward the left. Therefore, it's in the $$-\hat{i}$$ direction.

 Quote by Antiphon 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?

 Mentor Use thinner paper, write darker and/or use a light table.
 ... but what if I want to do it mathematically?

Mentor
 Quote by SammyS Seriously, Mr. Lewin clearly states that the force is toward the left. Therefore, it's in the $$-\hat{i}$$ direction.
Did you see this?

Recognitions:
Homework Help
 Quote by flyingpig The answer key said the force is $$-27\mu N \hat{i}$$ From Walter Lewin's method, I got $$+27\mu N \hat{i}$$. Help?