----- Original Message -----
From: "Savinainen Antti" <antti.savinainen@kuopio.fi>
I have taught intro relativity to HS students for a number of years. I can
assure that my students do not find it hard at all to understand the
experiment mentioned above without relativistic mass.
Here is how I do it: it is not a surprise to students that Newtonian
formulas do not hold when speed increases. Thus it is quite straight forward
to introduce the definition of momentum with the gamma factor and show that
it is consistent with the Newtonian definition when speeds are low. Then the
gamma factor is evaluated with different speeds and plotted against speed.
Its behaviour is quite obvious and "explains" why "momentum rises much
faster than velocity". There is no need to say that mass increases; the only
thing is to realize that the earlier definition of momentum is just an
approximation.
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Not sure I see how attaching gamma to the momentum is more 'satisfying' to
intro students than attaching it to the mass. We have two fundamental
concepts--mass and velocity which can be reasonably 'understood' from
everyday experience. When we combine them, we have a new concept--somewhat
more abstract in my mind, but I know others would set momentum as primary
here. That concept, momentum, can be pretty well 'understood' again from
experience--I stress that we have good instincts about momentum.
Now we go to the relativistic realm. Momentum doesn't increase linearly as
we would deduce from our Newtonian experiences, but we can directly measure
the velocities which are limited by the speed of light. I guess I have an
easier time (I try always to put myself in the mind-set of my students, and
maybe after 30 years of teaching ONLY intro physics, I myself am pretty much
stuck in that mind set ;-) with the idea that the mass is increasing in the
high speed realm rather than that the momentum was really a lot more
complicated in the low speed realm. But as someone else wrote--it
ultimately is a matter of taste, at least at the level we are discussing.