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In the figure for finding maximum elongation of spring why is this approach wrong-at maximum elongation,tension in rope=spring force,block stops at maximum elongation so finally tension in rope=weight of block .kx=T=Mg(M is mass of block) so x=mg/k?but answer is 2mg/k acc.to energy approach.

In the figure for finding maximum elongation of spring why is this approach wrong-at maximum elongation,tension in rope=spring force,block stops at maximum elongation so finally tension in rope=weight of block .kx=T=Mg(M is mass of block) so x=mg/k?but answer is 2mg/k acc.to energy approach.

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Grade:12

1 Answers

Vikas TU
14149 Points
4 years ago
Dear student 
If you use a constant force along the path, the spring will move past the position where F=kx, because it will reach that point at some speed. Thus it is incorrect to use the force method in the way you used it, because at maximal extension v=0 but a≠0. The energy method as you used it will give the correct answer. If, instead, the force is used to keep the spring elongated at rest, then the force method is correct
Good Luck 

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