Sorry for the bad drawing, but I hope that this will help you get a hold of the problem.
Consider an Atwood Machine with a total of two blocks, a mass less pulley, ideal string. One block rests on the floor, while the other one is at a height (H). Now, the string near the block that rests on the surface is slack. So, the other block falls freely, and later induces a jerk in the other block. How do I calculate the initial velocity of the two block just after the string is taut.
My approach was: 1) Calculate the velocity of the block in motion (initially) at the point when the is ALMOST taut. 2) Now conserve mechanical energy b/w the point where string was almost taut but impulse wasnt generated, and the point where impulse was generated, and the second block had JUST started motion.
But, my book says, I should conserve Linear Momentum between the same two points. I think this is wrong, because, The string that holds the pulley in place, will have an impulsive tension as soon as the impulse is generated in the string that joins the two masses.
According to you, what is right, and why?