Question about this problem and Walter Lewin's video

1. flyingpig

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

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

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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?

3. The attempt at a solution

2. flyingpig

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

Please...

4. Antiphon

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

5. SammyS

9,048
Staff Emeritus
@Antiphon: Nice answer!

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

LOL

Last edited: Apr 5, 2011
6. SammyS

9,048
Staff Emeritus
Seriously, Mr. Lewin clearly states that the force is toward the left.

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

7. flyingpig

If you flip the paper over, how can you see your work?

8. SammyS

9,048
Staff Emeritus
Use thinner paper, write darker and/or use a light table.

9. flyingpig

...

but what if I want to do it mathematically?

10. SammyS

9,048
Staff Emeritus
Did you see this?

11. Andrew Mason

6,896
How do you get the force going to the right?

AM

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