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Re: Newton's 2nd law lab



Regarding Rick Tarara's claim:
... We try it, and
low and behold, the slope (when the data is plotted appropriately) ends up
to have units of grams and a magnitude pretty close to the actual system
mass. We have 'discovered' the second law (rather than just verifying it)!
;-)

I think I'm missing something and don't follow you here. How does the
experimental discovery/determination that the magnitude of the
acceleration 'a' of the various parts of the apparatus is approximately
related to the cart mass 'm_c' and the falling mass 'm_f' according to:
(m_f + m_c)*a = g*m_f for some experimentally determined constant 'g'
having a value of about 1000 cm/s^2 either 'discover' *or* verify Newton's
second law? This setup has the two masses going in two different
directions and the resulting composite motion involves a messy
entanglement of the motions of system's center mass and the relative
internal positions of the system's constituent masses. Some contributions
to the total force on the system (e.g. the cart's weight, the normal force
from the track, and the force from the pulley) seem to be ignored. I
don't see what this has to do with N2 for this composite system--which
claims that the acceleration vector of the center of mass of a composite
system is directly proportional to the total of all the (usually external)
force vectors acting on the system with the proportionality constant being
the total mass of the system. If a particle (i.e. a single object which
is negligibly extensive in space) rather than a composite system is used,
then N2 claims that the acceleration vector of that particle is directly
proportional to the total force vector acting on that particle with a
proportionality constant which is the particle's mass.

David Bowman
dbowman@georgetowncollege.edu