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A simple pendulum has a bob of mass M. The bob is on a light string of length l. The string is fixed at C. At position A, the string if horizontal and the bob is at rest. The bob is released from A and swings to B, where the string is vertical. The tension in the string when the bob first reaches B is?

So to find tension I know that F(total) = Tension - (some other force), but I don't know for sure if that force is centripetal force or what.
To set up originally I chose Mg and know it is incorrect so I'm thinking that it is actually none of the above by using conservation of energy and knowing that centripetal force is mv^2/r where r in this case is l and v^2 is 2gh using conservation of energy where
mgh = 1/2mv^2
and then from there getting the formula. So the actual answer would be
m*2*g*h/l

does this sound correct?

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Kyle

  1. Oh wait I think I figured it out from a video.

    Total force is equal to mass time centripetal acceleration
    Total-F(c) = m*a(c)
    Where Total-F(c) = m* v^2/r
    T - mg = m * v^2/r

    where v^2 = 2gh and h = l and r = l
    so you get
    T-mg = m * (2*g*l)/l

    from that you get

    T-mg = 2mg
    then finally
    T = 3mg

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    Kyle

  2. I have no word about your helping me such a way
    Thank you

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    Andebet luelseged

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