Question about this problem and Walter Lewin's video

[flyingpig]

Homework Statement

https://www.youtube.com/watch?v=4zb0t4H0Kec

http://img4.imageshack.us/img4/7388/38804877.th.png

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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?

The Attempt at a Solution

 

[flyingpig]

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

 

[flyingpig]

Please...

 

[Antiphon]

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

 

[SammyS]

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

 

[SammyS]

Seriously, Mr. Lewin clearly states that the force is toward the left.

Therefore, it's in the $$-\hat{i}$$ direction.

 

[flyingpig]

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?

 

[SammyS]

Use thinner paper, write darker and/or use a light table.

 

[flyingpig]

...

but what if I want to do it mathematically?

 

[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?

 

[Andrew Mason]

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?

How do you get the force going to the right?

AM