Which Energy Law is the Key to Understanding Work and Conservation?

[mark2142]

Hi, everyone! There are a lot of work energy conservation laws and I get confused which one of them summarizes all of them. Which one I should keep with me and rest should be easy to derive on spot ?
1. $\Delta E_{mec}=0$
2. $\Delta E_{mec}=W_{ext}$
3.$\Delta E_{mec} + \Delta E_{ther}=W_{ext}$
4.$\Delta KE= W_{ext} +W_{c} +W_{nc}$
5.$\Delta E_{mec}=W_{nc}$

Which one?

Thank you for supporting.

 

[jbriggs444]

Hi, everyone! There are a lot of work energy conservation laws and I get confused which one of them summarizes all of them. Which one I should keep with me and rest should be easy to derive on spot ?
1. $\Delta E_{mec}=0$
2. $\Delta E_{mec}=W_{ext}$
3.$\Delta E_{mec} + \Delta E_{ther}=W_{ext}$
4.$\Delta KE= W_{ext} +W_{c} +W_{nc}$
5.$\Delta E_{mec}=W_{nc}$

Which one?

Thank you for supporting.

To me, they are all fairly obvious. You have a bunch of buckets where energy can show up. You may rule some out based on the situation. You may change which classification scheme you use to split things into buckets.

But as long as your buckets include all the places that energy can come from or go to and you are not double-counting anywhere, then you can write down an equation for energy conservation.

It's just book keeping.

Edit: I find the notions of "conservative" and "non-conservative" work to be wastes of time. If you have a potential associated with a force then you can use a bucket for the potential instead of a bucket for the associated work. The classification scheme I would use is "work that I am tracking using a potential" and "work that I am tracking as plain old work".

 

[mark2142]

It's just book keeping.

OK! Thank you.
BTW I think its the 3rd/4th eqn which is the mother of all equations.