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no...it's not true for every force in nature

take the example of work done by friction force.

can you first explain the question ? :)

|dw:1348895881344:dw|

ok

there are several forces in the nature..name them...whatever u know.

like electric force, magnetic force, spring force, friction force, gravitational force...

right?

yeah right

|dw:1348896083024:dw|

and these are all the closed loops

reply*

|dw:1348896151349:dw|

now suppose there is a ball situated at point A near the earth

okay

okay

|dw:1348896421255:dw|

yes

look for the angle between the force and the displacement

90

cos 90 = 0

yes..so work done by gravitational force( A to B)=0

okay

our objective is the work done by the gravitational force on the ball
so now we come to B to C

|dw:1348896858314:dw|

0 ?

i mean cos 0

displacement ( B to C) is vertically upward.

|dw:1348897246563:dw|

|dw:1348897411835:dw|

does it make sense for visualizing the angle between two vectors pictorially?

yes it does

so now we come to B to C|dw:1348897603389:dw|

got it?

force is vertically downward and displacement is vertically upward so angle between them is......?

okay cos 180

-1

yeah right

till now...it's fine?

yes :)

now work done by gravitational force (C to D)= 0
does it make sense?

yes it does

|dw:1348898023808:dw|

now we come to work done by the gravitational force on the ball (D to A)
tell me how much it'll be?

just look for angle between gravitational force and displacement when ball moves from D to A

cos 0 ?

yes

so work done by the gravitational force on the ball (D to A)= mg* h* cos(0)= mgh
BC=DA=h

okay

yes !

so total work done= W(A-->B) + W(B--->c) + W(C--->D) + W(D--->A)
=0 + (-mgh) + 0 + mgh
=0

so work done by gravitational force in the motion of a body over a closed loop is 0.

okay understood , and why is this not the same for every force in nature ?

but it's not true for every force in nature...e.g. friction force

and why ?

i don't understand , why not for frictional force ?

yes...I am coming there

okay :)

1st have a look...why not zero?|dw:1348899134273:dw|

like before , okay !

|dw:1348899311394:dw|

because during motion, angle between friction force and displacement will be 180'

am sorry but i did not understand the last part !

yes the same

90

|dw:1348899953137:dw|

|dw:1348900005470:dw|

see..if block moves from A to B then displacement of block =AB...right?

direction of displacement is to the right and the direction of friction force is to the left