Centripetal Force, Gravity and Normal

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

2. Relevant equations
[itex]F_{c} = \frac{mv^{2}}{r}[/itex]

3. The attempt at a solution

I think the normal force would be the magnitude of vector sum of centripetal force and gravitational force. So I did:

[itex]N = \sqrt{(\frac{mv^{2}}{r})^{2} + (mg)^{2}}[/itex]

However, it isn’t one of the answer choice. The actual answer is (c), and I have got no clue why it is (c).

I could imagine why answer is:
(a) Since mg is perpendicular to point Q, normal force is only [itex]\frac{mv^{2}}{r}[/itex]
(b) if calculated from the highest tip of the circle…

but where does [itex]2mg[/itex] comes from!?

Thank in advance.

http://ift.tt/1if24AH

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