Question

Work done by gravity!!!!.. i always have a hard time understanding this.. The work done by gravity in bringing mass m from infinity to point p SHOULD BE POSITIVE.. we all know that, gravity does positive work.. but if i try to derive it as shown it goes negative.. i have taken EVERYTHING into consideration, the signs , everything vectorially... i just don't get it.. ARrgggggg!!!!!!!!!!!

Answer 1

@chmvijay @ganeshie8 Help plzz :-/

Answer 2

i find this to be the fundamental problem in general when i do brute vector method of integration... i get work done from A to B and B to A to be the SAMMMEEE.. what is wrong.. plzzzzzzzzzzzzzz plzzzzzzzzzzz i neeed to know!!!!!

the B->A is supposed to be positive work.. but i can't figure out the fault!!! :( :( :( (: :( :(

Answer 3

Don't you dare leave without helping me orion!!

Answer 4

@Vincent-Lyon.Fr . need your help.. and i know if i do it in simple ways.. i get the right answer.. but i just need to know why can't i do it this way.. what is wrong in this way ..

Answer 5

It's because \(d\vec r=dr \;\hat r\) , not the opposite since your demo has been treated algebraically.

Answer 6

In your second example, it's the same:

\(d\vec r=d\vec x=dx\;\hat x\) in BOTH cases.

Answer 7

but in one case, my displacement is ALONG the direction of increasing r, and in another, it's along the direction of decreasing r, then why do we consider it the same?!

Answer 8

Of course it is not the same, but the limits of your integral are taking care of that.
Remember that if you swap the limits of your integral, the sign of the sum will be changed.

Answer 9

ohhww.. ok.. i get it.. i get it.. like always thanks a lot :) :) :) !!.. m pretty sure.. i do not wanna confuse my students with this.. :D