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Homework Help: 3 particles A,B,C of masses m,2m,3m lie at rest in that order on

  1. Oct 19, 2011 #1
    long questions but only need a little help i think:

    1) 3 particles A,B,C of masses m,2m,3m lie at rest in that orderon smooth horizontal table.the distance between each particle is a> A slack light inelastic string of length 2a connects A and B and an exactly similar slack string connects BC. A is projected in direction CBA with speed V,find the time whihc elapsesbefore C begins to move. Find the speed that C begins to move. show that the ratio of the impulsive tensions in BC and AB when C is jerked into motion is 3:1

    done first 2 parts, answers are 4a/V and V/6

    book no help with last part so i had a play and get the following:

    let tension between A and B be S tension between B C be T

    then F=ma gives

    for B: -S+T=2ma
    for C: T=3ma

    so -S+3ma=2ma giving S=ma

    so T/S=3.

    anywhere near correct reasoning to get the answer?


    2)

    a smooth plane is fixed at 30 degrees with its lower edge at a height a above a horizontal table. 2 particles P and Q, each of mass M are connected by a light inetensible string of length 2a,and P is held at the lower edge of the inclined plane while Q rests on the table vertically below P. the particle P is then projected with velocity u upwards along a line of greatest slope of the plane. find the impulsive tension in the string when Q is jerked into motion. determine the magnitude of u if Q just reaches the lower edge of the plane and the tension in the string while Q is moving.

    all i can get done on this is to find rt(u^2-ga)as the velocity of P when it has travelled a distance a.

    now i guess:

    now let velocity after jerk be V1

    have for Q: impulse=V1-o
    for P impulse =rt(u^2-ga)-V1

    so V1=0.5rt(u^2-ga)

    now tension is change in momentum of either of the particles so change in momentum of Q is
    m[0.5rt(u^2-ga)]-m[0]=m[0.5rt(u^2-ga)]

    now that is the correct answer but as in question 1 have i got there in a correct way?
    any help with the last parts ? not really got anywhere with them.
     
  2. jcsd
  3. Oct 20, 2011 #2
    Re: momentum

    If you could draw pictures, i could help you, i'm having trouble visualzing this
     
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