IIT JEE , AIEEE Forum - Physics , Mechanics - Work-Energy Problem

sanjbabbar

I want know that why conservative work is not zero even when the resultant force is zero???

What is potential energy???

edison

For non conservative forces the work done is non zero and thus depends on path of the body.

Further these are non conservative because of prescence of DISSIPATIVE forces like frictional force etc.

So the total force on the object sliding on the rough surface may be zero

as, Externally Applied force = - (frictional force)

But the work done is not zero as it is done to overcome the force due to friction.

Moreover work done is a SCALAR QUANTITY unlike force which is a vector,

so force can be zero but work done may not be zero.

sanjbabbar

I had asked about conservative work

phyana

in what contest did u encounter this ..............its something i havent ever heard.........but let me guess
conservative force means work independent of path
oh cant imgine new thihgs srysrysrysrysry

elessar_iitkgp

Resultant zero for a force system only implies that the forces balance. Until and unless the path in which the work is done is a closed loop, conservative forces will give some finite value of work. Only if the path is a closed loop, only then will the work done by the conservative forces will be zero, otherwise non zero.

kghedriu

well........the answer is like dis....

conservative work is work done by conservative force....

and net force is zero need not necessarily imply dat conservative force is zero..........

hence conservativ work need not b zero even tho net force is zero..

hope m clear...

thank u..

elessar_iitkgp

About potential energy ....

Lets take an example

Consider two masses kept at a distance from each other. Let one of the masses be fixed. The other mass experiences a gravitational attraction due to the fixed mass. If the free mass is not stopped, then it will accelerate towards the fixed mass and gain kinetic energy.

Now we will not let the free mass gain kinetic energy. Suppose an external agent applies a force equal and opposite to the gravitational force. Then the free mass is at equilibrium. Now suppose we move the free mass very slowly by an infinitesimal distance dx along the line joining the two particles, away from the fixed mass. The forces still approximately balance. (The gravitational force will change a little, but we may neglect it as the change is very small)

Now, the work done by the external force (against the gravitational force) during this small displacement does not increase the KE of the body, for the particle doesn't accelerate, So where does it go???

Now, work done always changes energy content. We define this work to correspond to the change in potential energy.
Hence
dW = F_ext dx = dU
As F_ext= F_gravdU = -F_grav dx

Or more generally,
dU =F_ext dx = -F_cons dx
That is change in PE is the amount of work done by an external force against a conservative force without changing the KE

suns_uday

resultant force is 0 means resultant effect is 0 but that doesnt mean that work is not done

so conservative work may or may not be 0depending on the displacement

srini

hey,

force is the resultant of all forces-both conservative,non conservative dissiptive ,

but conservative work includes only the those for c es -like gravitational,other field .like say gravitational work-whether you take an object to height through different or complicated way still the energy is the same only. It doesnt depend on the path you choose

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