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On 03/17/2010 12:24 PM, LaMontagne, Bob wrote:
The term mgt is rarely seen explicitly - and I have never seen it
refered to as momentum.
That's an interesting observation. It initially took me
by surprise, but I'm convinced it is a valid observation.
I need to be reminded every so often that the way I do
things is not the way everybody else does things.
I read the PSSC Physics book at an impressionable age.
It says quite clearly that Δp = F Δt ... and I always
understood that to include Δp = mg Δt as one contribution
... but the book doesn't actually say Δp = mg Δt. The
book applies conservation of momentum more-or-less
exclusively to collisions that are (a) impulsive and
(b) horizontal ... in contrast to mg Δt which is steady
and vertical. Hmmmm.
I checked some other books, and they're all worse.
There are a jillion kinematics problems that can be
formulated either way: either in terms of conservation
of momentum or in terms of force balance ... and I agree
there is a widespread tendency to emphasize the force
approach to the near exclusion (or total exclusion) of
the momentum approach. This is IMHO a scandal, but it
is the situation we face.
In addition to the problems that can be done either way,
there are some problems where the momentum approach is
clearly easier and better. This includes relativity
and includes fluid dynamics (not to mention relativistic
fluid dynamics).
There are some statics problems, notably PVW problems,
where connecting the force to momentum is not helpful,
but PVW is just conservation of energy, so the problem
revolves around a conservation law either way.
My father impressed on me the importance of conservation
laws when I was about 5 years old, and I've never learned
to look at statics or kinematics problems any other way.
The term mgt is rarely seen explicitly - and I have never seen it
refered to as momentum.
I don't disagree ... but as for me personally, I am quite
unable to see mg Δt as anything *other* than momentum
transfer ... and I see no reason to change my ways.
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