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the starting premise is wrong.
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So, one very useful lesson to learn here is
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when you apply formula,
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don't forget the conditions under which you derived it,
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and don't apply the result to a case
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where the answer does not belong to the domain
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that you assumed you're applying it to.
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So, the trouble with friction is the following.
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Friction is not a definite force with a definite magnitude or direction.
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The magnitude is definite,
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but direction is not definite.
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It is uphill if you're going downhill;