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Re: Newton's 2nd law lab



The experiment is normally done to confirm N2, in that the weight of the
falling mass is the applied force, the mass of the system is the mass of the
cart plus the falling mass, and the accelerations are calculated from
measurements of time and distance. Plots of acceleration versus weight
produce linear graphs such that y=mx becomes F = ma (the slope is 'm'). The
variation on the experiment is to NOT let students know the numerical value
of the Force.

David objects to ignoring the Normal Force of the track or the pulley force.
The Ealing air tracks have an 'air pulley' that lets recording tape float on
a thin cushion of air--thus producing a nearly frictionless pulley. The
Normal force of the track is perpendicular to the direction of motion and
therefore does not contribute to the acceleration of the cart.

This is a Lab for INTRODUCTORY PHYSICS STUDENTS. As seems often the case, I
think David is too concerned with here with complicated details of 'real
world' physics. However, this experiment, in many different, forms DOES
produce results quite consistent with the typical modified Atwood's machine
analysis WHICH relies on an application of Newton's Second law to analyze.
I really don't see the problem!

My twist on the typical experiment makes the analysis a little more
interesting than the typical "OK, we know F = ma, so let's measure F, m, and
a and see if indeed F = ma." The assumption that the force of the falling
mass IS proportional to that mass does severely weaken the 'discovery' part
of the experiment, although the students don't realize that, but that's why
I included a ;-) at that point in the description.

Rick


Howdy,

The problem is that we understand Newton's Second Law and believe it and
the combined mass of two objects moving in two different directions comes
by combining the 2nd Law equations for each object. This is not at all
obvious to many others (including that parent!) and certainly not to those
folks we are trying to get to understand Newton's 2nd Law.

Good Luck,

Herb Schulz
(herbs@interaccess.com)