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Homework Statement
Hello there!
I'm having a big issue with understanding my books explanation. The image is taken from my book. The book is trying to prove the formula F = -k*y. This is the chapter ( Simple Harmonic motion)
They got the following equations(Explanation from book):
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ks0 = mg
k(s0+s1) = ks0 + ks1
F = ks0 + ks1 - mg = ks1, because ks0=mg
''But the y-coordinate is negative, and equal to -s1. Also F can be written as F = -ky''
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Image:
Homework Equations
I can't understand this at all. IF I define positive direction as upwards, and negative as downwards, just like they have done I suppose, how isn't the s0 and s1 negative because the spring is obviously stretched downwards?
Shouldn't s0 be a negative value? Because for example if the starting position is 10cm, and it moves to the ''Jämviktsläge=zero level'') then it should move 0-10 = -10cm from its starting position, in other words; it's stretched -10cm?
The same with s1, if negative direction is down, then it should move -2-(0) from the resting position is this correct?
However I get totally wrong equations, unlike theirs.. I've been trying for 4 hours now..