Re: Newton's 3rd law? was Re: inertial forces (definition)

[Leigh Palmer <palmer@SFU.CA>]

> Do you actively work to dispel the idea that weight is caused by gravitational
> force in your intro class?

No. I tell them that weight in an Earthbound laboratory is *mostly*
due to Earth's gravity, but that the cause doesn't matter; we are
only constructing a model, and it is an excellent model representing
the mechanics in the Earthbound laboratory.

> Do your students understand the subtleties of your
> language choices or are you just being careful so as not to be responsible for
> misconceptions at a later date?

I comment on the importance of language every time I feel it is
useful to do so, and that is fairly often, though I don't do it in
every lecture. I do make it very clear when I am defining a term
which will be used in the development (like energy or efficiency)
and I tell them to put all common usage meanings away when they
are doing physics and to use only the defined meanings.

> Do you connect the downward motion of falling
> objects with the circular motion of orbits or is that not part of your
> study of
> Newtonian physics in the Earthbound laboratory and saved for as you said
> later on?

When I introduce gravity I tell them about Newton's brilliant
insight that the Moon is falling under the influence of the same
force that would make a rock hurled sufficiently rapidly from a
mountaintop circle the Earth. I assume everyone uses that one. It
should not be omitted from any introduction to gravity.

Leigh