Professor Dave here, let's learn about
work and energy.
Just like force, the words work and energy have a
variety of connotations in common language. When we
say work, sometimes we mean exercise
other times we mean a place we go every
day and do something to earn money.
Energy is even more convoluted. We might
say that today we have no energy, meaning
we feel tired, or we might use the term
in a completely metaphorical way, saying
that a person or a location has good
energy. But once again, in physics, these
words mean something very specific, and
if we are to learn science we must allow
the rigorous definitions to replace the
colloquial connotations. Work is defined
as action done on an object whereby an
applied force causes a displacement of
that object. A basic example would
involve pushing a box 10 meters across
the floor. Work is equal to the magnitude
of the applied force times the
displacement of the object, so it has
units of force times distance. If you
push with a constant force of 100
Newtons over the 10 meter distance you
would do 1000 Newton meters of work
which are equivalent to another SI unit
called joules.
This means that work can only be done
when displacement occurs. If you push a
brick wall with the same force for the
same amount of time as the box
you may get very tired but technically
you have done zero work. In addition, only
the components of the applied force that
are parallel to the displacement are
considered to be contributing to work.
If you push the box with your arms parallel
to the floor, all of your effort
contributes to work done on the box, but
if you are pushing at a downward angle
only the x-component contributes to work
while the y component does not. In such a
scenario, instead of using W = FD we
could calculate the work done on the box
using W = FD cosine theta, as this
will give us the portion of the
applied force that is operating in the
horizontal direction. If theta is zero
meaning the applied force is horizontal
cosine theta will be equal to 1, leaving
us with W = FD, and if you were to
push straight down on the box theta will
be 90, and cosine theta will be zero
meaning no work is done on the box and
it will not move. Any angle in between
and there will be some motion. We must
bear in mind that work is a scalar and
it can be positive or negative. Work will
have a positive value when the applied
force is in the same direction as the
object's motion, like when pushing the
box, and work will have a negative value
if the applied force is in the direction
opposite the objects motion, like if your
baby brother pushes the box the other
way but without enough force to stop it
from moving forward. The sign on work is
very important because it tells us if
the speed of an object will increase or
decrease as a result of applying the work.
Energy, another elusive term, is
quite simply the capacity to do work, and
this is summarized in the work-energy
theorem. This states that when work is
done on an object there will be a change
in that object's kinetic energy, which is
the energy of motion.
This also means that an object in motion
is able to do work on another object.
The work-energy theorem brings together
concept from Newton's laws and
kinematics to describe motion in a
way that we will examine further in the
next clip. For now, we must simply purge
ourselves of any prior connotations of
the word energy and understand that it
merely describes a property of an object
or system that can be transferred into
another object or system, often in the
form of work. There are different kinds
of energy, including kinetic and
potential energy, thermal energy, chemical
energy, and more, and we will discuss all
of these when the time comes.
Once again the SI unit for energy is the
joule, which is defined as a Newton meter
even though energy is not always
transferred in the form of work. But our
understanding of energy will grow as we
examine the various types of energy, so
let's move on to that part of the course.
Thanks for watching, guys.
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