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

1. Oct 19, 2011

### jiboom

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. Oct 20, 2011

### Liquidxlax

Re: momentum

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