3NEONE PLS REPLY.
i was really confused abt work done by friction.
1)Some authors says dat it is just like a workdone by a conservative force so the expresion is Wfriction=Frictional force*displacement
2)Some says dat it depends on path so the Wfriction=Frictional force*distance.
But wat i felt correct was
3)K.Echange-Workdoneconservative=Wnonconservative
if ne one accepting 1) or 2) pls post ur comments in this thread.
and sry for the ones if u felt dat this post was an useless discussion.
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9 Answers
1Hmmmmmmm....,nice thread,msp.
well,according to work-energy theorem in a compact form,
Wconservative+Wnon-conservative+Wexternal=Kf - Ki ,
WHERE Wconservative + Wnon-conservative represent total work done by internal forces.
Wfriction comes under Wnon-conservative.
And when frictional force does work,we can write its work done as:
frictional force * displacement.
but its= frictional force * distance,only when diplacement=distance
62Think of it like this...
work done by all conservative forces is change of kinetic energy...
why?
Because
F.ds = dW
dW=ma.ds = mvdvdt
integrating we get W=ΔKE
Now net work done is work due to conservative + work due to non conservative forces
Wconservative+Wnon Conservative = Δ KE
1very true sir. moreover,it is the systematic derirvation of work-energy theorem
3sir i know work energy theorem and i accept dat,but i don want the derivation sir,what i need is the explanation for 1) and 2).
i have seen a lot of pbms using 1) or 2).one or more pbms in Fiitjee's buk,irodov's have solvd using dat.
i think u may also have seen in buks.
3when a pbm is askd in jee which shud i follow,some times if we use work energy theorem we need to have one more eqn to solve.But in some qns there is lack of equation to solve dat one.
62if you dont look at what i have written in #4 as a derivation but an explanation to what you have asked, then i think you will get both your answres..
106the best way to find out work done by friction is by using WE theorem:
Wfriction + ΣWother forces = ΔKE
So if you know the remaining terms then you can find the work done by the frictional forces
If the frictional force is constant in magnitude and the point of contact does not remain stationary wrt the surface. then Wfriction is simply ∫F.ds If F is constant then it is Fs.
This method cant be used in partial rolling motion (when the body is slipping as well as some rolling) there U have to use WE theorem to find the work done by friction