Two balls having masses m and 2m are fastened to two light strings of same length l (figure 9-E 18). The other ends of the strings are fixed at O. The strings are kept in the same horizontal line and the system is released from rest. The collision between the balls is elastic. (a) Find the velocities of the balls just after their collision. (b) How high will the balls rise after the collision?


Given that two balls having masses m and 2m are fastened to two light strings of same length l,


Let the velocity of m reaching at lower end is V1,


From work energy principle




Similarly, velocity of heavy block will be



Therefore,



Since, both the balls acts in opposite direction.


{a} Let the final velocity of m and 2m is v1 and v2 respectively.


According to law of conservation of momentum




_________ (1)


Again,




_____________ (2)


Subtracting equation (2) from (1)





Putting in equation (2)







Hence, light ball will have velocity towards left


And heavy ball will velocity


{b} Let h be the height gone by heavy ball


Putting work energy principle





Similarly, h’ be the height reached by light ball





So, the balls will rise of following height after the collision


Light ball and heavy ball.


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