From the viewpoint of a first-year student:
N2 tells me that a net external force is needed to
make a mass accelerate (and that the converse is
true). I observe a car accelerate from rest; I
conclude (from N2) that a net external force acted on
it. I also observe that the KE of the car went from
zero to some finite value (in the road frame). I
therefore conclude that the net external force
(whatever it was and wherever it came from) was
responsible for the change in KE. In the road frame,
the car has experienced a delta v which shows up as
both a delta KE and a delta p.Is this analysis (on the
part of my mythical first-year student) not correct?
The issues of where the KE "came from" and how exactly
that net external force arose are more subtle and will
be very difficult for most HS students to comprehend.
John Barrere
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