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The energy in the book is a very bad answer, although it is commonly used
by
many teachers. The energy should be said to be in the book-Earth system.
For students who keep on asking about this, the idea of it being in the
gravitational field is also reasonable. They can picture it like a rubber
band which is storing energy as it is stretched. The only problem with
this
answer is that the general gravitational law has the force becoming weaker
with distance, which is the opposite of a rubber band. One way of
picturing
it is to think that there is always a connection, but it just gets thinner
as the two objects separate. Get the students to think of stretching the
connection between the two objects.
Modeling uses the idea that you can picture where the energy resides, and
never uses the term type of energy or energy conversion. Essentially to
help students understand that energy is conserved it has been found to be
helpful to get rid of of energy transformations and get students to label
energy as to where you put it. So the idea of it being in the field can be
helpful. Is it really necessary to know where the energy is located?
Probably not, but students find it helpful to have a place to localize it.
The developers of Minds on Physics also stress the ide of the energy being
in a system.
The system approach is very important, but is ignored in most standards.
So
many standards say that students should consider that fuel contains energy,
but that is nonsense because you can not extract energy from fuel alone.
The energy is in the fuel-oxidizer system (fuel-air). The idea of energy
being in fuel also feeds into the misconception that bonds contain energy
so
when you "break" them you get energy back. Any chemist can tell you that
when you "break" bonds you always put in energy, but biologists are prone
to
the misconception because it has been repeated in many biology books.
There
is a Texas physical science book which says in one chapter that you put in
energy by breaking bonds, but in the bio chapter it says the opposite. The
better terminology is to say you stretch bonds. The idea of a connection
that acts like an elastic object helps students to understand bonding as
well as gravitational energy. I favor hitting the students with all local
forces at once rather than in separate chapters. This is done in Minds on
Physics to good effect. The also juxtapose the generaly gravitational law
with the electric and magnetic force laws.
So the Finnish book should really stress the idea of energy being in a
system. I suppose it is alright to say that kinetic energy is in the
moving
object, while potential energy is in the Earth-object system. Elastic
energy is in the stretchy object. Of course kinetic energy is relative to
the reference frame, but for the initial conception is probably OK to put
it
in the object.
The problem with the mass increase is that it is not really detectable.
Physics already has the problem that many of the things we discuss can not
be seen. Lawson has shown that you need to be above the formal operational
level in your thinking to be able to reliably handle things that you can
not
see. Students can not see the gravitational field either, but I think
visualizing a connection between the objects might be more reasonable than
visualizing an increase in an abstract quantity called mass.
Analogies are one of the ways that we think, so they are extremely valuable
in getting students to "understand" concepts.
John M. Clement
Houston, TX
For the purposes of first-semester high-school physics, one
can imagine that lifting a book imparts an energy E = m g h
that resides "in" the book. However, even back on Day One of
modern science, Galileo was quite aware that such a simple
approach was valid only for laboratory length-scales, and
could not be applied to planetary length-scales or beyond.
By second-semester high-school physics we discuss Newton's
law of universal gravitation, in which the book and the
planet play symmetrical roles. You could say the energy is "in"
the planet rather than the book, and it would make equally
much sense, i.e. no sense at all.
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