Physics 1 Topic 5: Work done by Force (scalar F·Δx) Transfers Energy (scalar)
. . . Energy perspective is mostly about place rather than time
. . . Work is done to an object as the Force (its contact point) displaces ... (cf: Impulse done as Force endures time-wise)
. . . Force that is function of location can be treated as Potential Energy (function of location)
. . . a Potential Energy abbreviated PE or U stores Energy due to its condition (object in environment)
a Force does positive Work if the displacement projection is along the Force (similar direction)
. . . a Force does negative Work if the Force projection opposes the displacement ("opposite" direction)
. . . a Force does zero (0) Work if the displacement is perpendicular to the Force.
motional Energy : accumulated momentum, weighted by its velocity has Kinetic Energy KE = ½ m v² . . . always positive !
local gravity : Fgrav = m g has PEgrav = m g h . . . gravity down, so height measured upward (from arbitrary "zero level").
pressure Force : Fpress = P A has PEpress ≈ P V . . . roughly ... (should measure outside Volume)
spring Force : Felast = − k s has PEspr = ½ k s² . . . always positive !
general gravity: FS = mp GMS / r² has PES = − mp GMS / r . . . always negative !
. . . there are only 3 conservative Forces this semester, so it is not a long list (local grav is an approx to "G" grav)
rope's Tension Force always does zero Work itself (Force is along rope length, but length does not change)
. . . but can transmit Work along the rope, if the rope attachment place moves (parallel rope) .
friction Force : Ffr = μ F_|_ always does negative Work (or 0), so has no formula for PE
. . . you must use Wfr = Ffr · Δx . . . same for viscous Force and drag Force.
. . . these forces transform KE into Thermal Energy ... yeah, raising their Temperatures ... wait till Topic 9.
Important statement #6 about how the Universe works : Energy is Conserved
. . . each Force from an external subject can do Work to the system ... if the Force pierces the system boundary (bag) ;
positive (+) W transfers Energy from the subject (which applied that Force) into the object (system).
=> KEsystem,start + Σ Wby all F = KEsystem,end . . . this statement is called the "Work-Energy Theorem".
the 3rd Law for Forces says that , if the Force from me to the rock does positive Work , then the Force from the rock to me does the same negative Work.
. . . every Force has a 3rd Law pair-mate , so the Energy simply gets moved from item to item by those pairs.
. . . it is often useful ("easiest") to consider what Work the applied Forces might potentially do ;
and ascribe that quantity to the system as Potential Energy ... with with this perspective
a Force's (+) Work transforms its Potential Energy into some other form of Energy, as it is actualized (made real rather than potential).
=> Σ Estart + Σ Wby F not in PE list = Σ Eend , for a system . . . that's the way we want to do it.
. . . Examples . . .
