hi.i have searched for an inertia formula but could not find anything.after that i thought that maybe there is not.and now i want to ask you a question.assume that we have a large mass (which has a large inertia )witch is connected to a spring.we launch another mass to the spring.and then i want to know the length of spring .
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The question is not properly phrased I guess. The length of the spring has got nothing to do with what is connected to it. But in case you are asking the compressions/ elongations in the spring, then more data needs to be provided.
Data provided are not sufficient to answer your problem.To find out the elongation you have to be provided with the spring constant and the masses hung.