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Homework Help: Circular motion

  1. Nov 6, 2007 #1
    A mass M attached to a string of length L moves in a circle in a vertical plane.AT the top of the circular path, the tension in the string is twice the weight of the ball.
    --find magnitude and direct of Net force and speed of the ball at the top.Express aswer in term of M,L,g

    net force= T-mg = mv^2/L
    2mg-mg = mv^2L
    F=Square root gL

    Square root gL=mv^2L
    Square root of( (L*square root Lg)/m )
  2. jcsd
  3. Nov 6, 2007 #2


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    They want the magnitude and direction of the net force in terms in terms of m, g and L... so v shouldn't be in your answer...

    When the ball is at the top of the loop T and mg are in the same direction:

    Fnet = T + mg = mv^2/L

    so what is Fnet in terms of m and g... what is the direction of Fnet. What is v in terms of m, g and L...
  4. Nov 7, 2007 #3
    Fnet = T + mg
    2mg+mg=3mg , the direction for Fnet is toward the center? not sure

    L3mg=mv^2 m cancel
    v= Square root of 3Lg, the direction for velocity is horizontal and tangentially to the circle, to the right
  5. Nov 7, 2007 #4


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    yes, v=sqrt(3Lg) looks right to me. Yes, direction of Fnet is towards the center... in other words downwards since the mass is at the top of the circular path.
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