IIT JEE , AIEEE Forum - Physics , Mechanics - forces--doubt

tarun_bits

In the following figure the surface is frictionless...

We have to apply the conservation of mechanical energy principle...is the gravitational force an internal force or external force.....]

if it is an external than we can conserve energy because it is balanced by the normal reaction force......

plz clear the doubt

karthik2007

Yes you can, note that N does not do any work, because it is always normal to the surface, while mg is not always normal to the surface, hence it will do a positive magnitude of work.

Decoder

look... u have nothing to do wid normal reaction here...it will always be balanced...( dimensions...of curved path aren't given)...the main thing is..no non-conservative force is acting on the system..thus.. its total energy is not altered...at all..

tarun_bits

u both are right kartik and decoder...but it is still not clear to me that the gravitational force is external or internal to our system..

tarun_bits

anybody please..

iitian_aakash

if we take earth + body as a system g.f. is internal force. use this and write the relation. as acc. of earth is neglible there is no change in its kinectic energy. so finally you will get same answer.

karthik2007

Here's a simple way to conserve energy:

Now, consider a body moving down a rough inclined plane. Its initial energy at the top was mgh, final energy at the bottom is 1/2mv^2.. but it will be less than mgh (coz of friction), right?
here, friction is the culprit, so while conserving energy, simply add work done by friction to the final KE to compensate.
mgh = 1/2mv^2 + f.S

do the same for other forces... make sure you find out where they are to be added

tarun_bits

you have indirectly told the work energy theorm......mine ques is this......

wE CAN TALE IN THE ABOVE FIGURE GRAVITATIONAL POTENTIAL ENERGY ZERO AT ANY POINT...RIGHT...SO IN THAT CASE WILL mg BE AN INTERNAL OR EXTERNAL FORCE....

karthik2007

Dude, force due to gravity is external here

tarun_bits

so in this case gravity did the workk..

and you must hav read that if the external forces are present we can`t conserve mechanical energy??

karthik2007

dude, here, we are using the work energy theorem!!!

generally, we call it as conservation of energy in cases where only gravity acts

ambareesh13

simple yaar !!!
you dont need to think that gravity is external or internal just remember the work energy theoram says the work done by all the forces on a system "(internal or external ;conservative or non conservative )" is equal to the change in kinetic energy of the system

ambareesh13

dont mess up you mind with conservation of energy just always go the basics or you will be always confused

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