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General question regarding positive and negatives in equations?
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[QUOTE="haruspex, post: 4521155, member: 334404"] It depends what you mean by 'g'. If you mean 'the acceleration due to gravity' then it is just g, but its value is either 9.81 ms[SUP]-2[/SUP] or -9.81 ms[SUP]-2[/SUP], depending on whether you measure the vertical direction as positive downwards or upwards. Similarly the height h. So the PE is always -mgh. If an object mass M is 10m above the baseline and upwards is positive then h = +10m, g = -9.81 ms[SUP]-2[/SUP], the PE is -Mgh = +98.1M m[SUP]2[/SUP]s[SUP]-2[/SUP]. (Alternatively, you can regard g as standing for +9.81 ms[SUP]-2[/SUP] then say the accn due to gravity is +g or -g depending on whether down or up is positive. I'm not at all sure whether one of those two views is standard.) Similarly for the KE, it doesn't matter whether v is positive or negative, v[SUP]2[/SUP] is always going to be +ve. For force*distance, again, the signs matter. Distance and force are measured in the same direction, and work done by the force is F d. If you push on a block with force +3N but someone else pushes harder from the other side and you end up retreating 1m then the distance is -1m, so the work done by you is -3J. So in none of these cases is it true that we just take the absolute values. The sign comes out correctly by applying the rules. I'm not sure what you meant by the reference to cos theta wrt W. What you do have to watch out for with 'work done' is whether it's work done by (whatever) or work done on that. In my pushing example, the work done by you was -3J, which is the same as saying the work done on you was +3J. I hope this is clearer now. [/QUOTE]
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General question regarding positive and negatives in equations?
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