(WITH LESS WORK)
The traditional approach to work and energy in a standard physics course often ends up being a
rather imprecise, confusing mass of equations and definitions, such as "energy is the ability to do
For example, the work-energy theorem is a point-particle model that is
often inappropriately applied to situations that require the consideration of internal structure, and
the 1st Law of Thermodynamics is rarely used to analyze mechanical systems, despite its universal
This unit focuses on
, defined as a conserved, substance-like quantity with the capability
to produce change.
Work is de-emphasized, and is more accurately called "working", indicating
the nature of "work" as a
of transferring energy into or out of a system via external forces.
The 1st Law of Thermodynamics is used as the primary means of analysis of mechanical systems
because of its fundamental, universal nature.
All energy interactions can be characterized as energy
, depending on how the system is defined. Energy storage modes are kinetic, potential and
internal energies, designated as ∆E with corresponding subscripts (∆E
Energy transfer mechanisms are working (W), heating (Q), and radiating (R) . As
awkward as it may be at first for the physics teacher to refer to W as “working”, the gerund is
deliberately chosen to emphasize the process of energy transfer.
The relationship between energy storage and transfer is shown by the 1st Law of
Thermodynamics, ∆E= W (+ Q + R). This is shown by the system schema below:
It shows that energy transferring into and out of the system affects the nature of the energy storage
in the system.
The 1st Law of Thermodynamics and the Law of Conservation of Energy state that
the algebraic sum of these energy changes and transfers must add up to zero, accounting for all
changes relative to the system.
This crucial concept is incorporated into the pie chart and bar
graph representational tools used in this unit.
The power of using the 1st Law of Thermodynamics for analysis is that it makes it possible to
take into account the internal structure of the system, since energy dissipated by frictional forces
) can be accounted for as energy stored internally in the kinetic and interaction energies of the
particles that make up the objects in the system.
In its expanded form,
the 1st Law of Thermodynamics is W + Q + R = ∆E,
where ∆E = ∆E
So for mechanics (in this unit), neglecting Q and R)
Notice that when the internal structure of the system
be ignored, the work-energy theorem
appears naturally, from the 1st Law: ∆E
= W assuming no other storage modes are involved.