[[Work Energy and Power]]

# SUMMARY
* System of one object can only have KE.
* Mechanical energy is just KE + PE.
* If work = 0 and no no conservative forces then ME = 0
* If work is done then systems total energy changes
* Choice of system boundary decides if an interaction is internal or external (no shit)
* Nonconservative forces like friction or air resistance can dissipate mechanical energy as thermal energy or sound.

## Mechanical Energy Components
* Kinetic energy is energy of movement
* Potential energy is energy due to position or config
* Total ME stays the same when only conservative forces act inside the system

By choosing the system carefully, you can set up systems where even if things in the system change no energy exits of enters, called Constant Energy Systems.

When a system energy changes, that change matches the change across the system boundary. Energy entering increases total energy, energy leaving decreases it, and net change equals sum of in and out.

## System Selection and Energy Changes

|Comparison|Falling ball as system|Ball–Earth system|
|---|---|---|
|System contents|Ball only|Ball and Earth|
|Gravity|External interaction; does work on the ball|Internal interaction|
|Gravitational potential energy|Not included|Included|
|Energy equation|ΔK=WgravityΔK=Wgravity​|Ki+Ug,i=Kf+Ug,fKi​+Ug,i​=Kf​+Ug,f​|
|Mechanical energy conserved when air resistance is negligible?|No; the ball’s kinetic energy changes as energy transfers into the system|Yes; energy converts between kinetic and gravitational potential energy|