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Re: Why work before energy in texts

Jim Green wrote:

There can be no change in the level of energy of a system
without doing work on the system.

Level usually means elevation to me. Why not the "amount"
of energy? In either case the "what is energy?" question
must be addressed in the first physics course. You must
define new concepts when you introduce them. Right?

Let's call 0.5mv^2 the energy of the fligit and the force
times the distance the work I did on the fligit. Class after
you take the calculus, you will understand how this is derived.

It is not necessary to say "the work I did", the "work done by
my force" would probably be better. A work done be a person
is often associated with "being tired", not with the physical
quantity we call work in physics. I agree, it is a nitpick.

You do not need calculus to show that 0.5 is not an arbitrary
factor. This is not a nitpick.

Greater work; greater increase in the level of the
energy. ie W==Fxd = deltaE -- always and necessarily

"Always and necessarily?" I would prefer to show that this is true
in one or two situations before generalizing. You probably had
this in mind, right?

Now if I slide the book across the table slowly, there are again two
forces acting -- my hand and friction -- and the work done by each force
cancels. See the book does not continue to move -- the level of energy is
not increased.

Another nitpick. The amount of energy associated with that book
would not necessarily change if the book continued to move. Think
of the constant velocity and of the horizontal path. Yes, I know
that you know this, Jim.

What is the difference between gravity and friction? With gravity I
can get a subsequent increase in the level of energy and with friction
I can not. Let me call such forces conservative and non-conservative.

Because the work done by conservative forces has the potential to increase
the level of energy of the system, I will call this work, deltaPE. And I
will call the first energy KE.

Are you saying that deltaPE = m*g*h BY DEFINITION or are you
referring to a hidden derivation?

So Wnc + Wc== Wnc + deltaPE = deltaKE

In most textbboks this is derived from F=m*a plus kinematics.
Are you leaning on this?

But things look better if I change signs and write

Wnc=deltaKE + deltaPE

Algebraically, it must be - deltaPE. You probably meant

Wnc=deltaKE + something, where something=-deltaPE

And we have work and both PE(-Wc) and KE(E) all at
the same time. And the students understand what is going
on and why much better.

How does the "at the same time" promote better understanding?
This is not a nitpick.