When is the total mechanical energy not necessarily conserved?

A. when friction is present

B. when the mechanical energy is entirely potential energy

C. when the mechanical energy is entirely kinetic energy

D. Total mechanical energy is always conserved in any closed system.

***Not sure!! Please explain? Thank you:)

Question

When is the total mechanical energy not necessarily conserved?

A.       when friction is present

B.       when the mechanical energy is entirely potential energy

C.       when the mechanical energy is entirely kinetic energy

D.       Total mechanical energy is always conserved in any closed system.


***Not sure!! Please explain? Thank you:)

anonymous · Answer

https://answers.yahoo.com/question/index?qid=20121218002603AAWmMVt

anonymous · Answer

so it would be either choice b or c? :/

aaronq · Answer

i'm not sure to be honest

anonymous · Answer

ohh okay, thanks though!!

michele_laino · Answer

I think when we have a closed system, since by definition, a closed system is a system which can not change energy with other system

michele_laino · Answer

systems*

michele_laino · Answer

here we have to take in account the definition of closed system

anonymous · Answer

ohh so it would be choice D? because they cannot change enemies within each other? :/

amistre64 · Answer

NOT necessarily conserved
^^^^

anonymous · Answer

sorry *energies hahhaa

anonymous · Answer

@amistre64 what do you mean by that? so it wouldn't be a closed system? :/

amistre64 · Answer

what is the definition of mechanical energy?

anonymous · Answer

the ability to do work?

amistre64 · Answer

thats not the definition i get

michele_laino · Answer

hint:
empirically, when there are friction forces, we have the subsequent condition:
KE=PE
What we have to do is to establish if those friction forces belong or don't belong to our system

anonymous · Answer

ohh i'm not sure then @amistre64 is it potential energy + kinetic energy? :/

and @Michele_Laino so they are interchangeable ?

amistre64 · Answer

Me = PE + KE

PE and KE are conservative energies in a closed system

3 of these define conservation, one does not.

michele_laino · Answer

If we consider the gravitational field, which is a conservative field, then the sum:
kinetic energy+potential energy of a body has to remain constant

anonymous · Answer

and so the one that does not would be my solution? :O

amistre64 · Answer

what eats at energy?  what makes energy go away (be not conserved) ?

its asking for which one is not necessarily conserved, yes

anonymous · Answer

work makes energy go away? because it is being used?

amistre64 · Answer

is work one of your options?

anonymous · Answer

would it be choice B then ? :/

amistre64 · Answer

when is PE not conserved?

michele_laino · Answer

In general friction forces are viewed, as the weight force, like external forces acting on a defined system. The reason is simply:
if we include the friction forces into our system, then we have to consider that part of mechanical energy will leave our system as heat

anonymous · Answer

ohh when friction is present? so choice A would be the solution? :O

amistre64 · Answer

friction destroys, when things are destroyed they are not kept the same ... the are not conserved.

amistre64 · Answer

not saying its right, just saying thats what id go for

michele_laino · Answer

If we include friction forces into our system, then our system will be  not a conservative system

anonymous · Answer

ohh okay, so then my answer is that total mechanical energy is not necessarily conserved when friction is present? :O

michele_laino · Answer

that's right!

anonymous · Answer

yay!! Thank you both @Michele_Laino @amistre64 :)

michele_laino · Answer

Thank you! :)