Don't "mechanical advantage" disagree with the law of nature ? hello I just can't swallow the idea of mechanical advantage. * if you have a 10 KG mass , you need , say , 100 N to left it up * if your friend helps you , each one of you will exert only 50 N * but if you are alone , and you have two pulleys in a distinct configuration , then - again - you need only 50 N I just want to ask : in the third case , who is the "friend" that exerts the other 50 N ?? from where did that energy came from ?? isn't energy "cannot be created" ??