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Re: Newton's 2nd law Lab



We do a similar Atwood lab in two ways. First, we do a series in
which the sum of the masses on the left and right sides of the pulley
remains the same, but the difference changes. Second, we do a series of
measurements in which the difference in masses is constant, but the total
mass changes. The first part shows the dependence on the net force, the
second the dependence on the inertial mass. We are not using force
sensors in the present experiment. The main problems we encounter are the
friction in the pulley, the rotational inertia of the pulley, and timing
uncertainties. My recollection is that if we use spark tape or a sonic
ranger to measure the motion, that we find non-constant accelerations.
Perhaps the pulley isn't round!

The air track experiment you describe is the subject of discussion in
Arnold Aron's book. If the weight of the hanging mass is taken to be
the force, then the inertial mass of the hanging mass is also
being accelerated and keeping the total mass constant simplifies the
analysis. (Since the mass on the cart is usually hundreds of grams, and
the hanging mass is often only a few grams, the quantitative error that is
often made by neglecting the inertial mass of the hanging mass is not
large, but the conceptual error is nasty!) If a force probe on the cart is
used, the force it measures will be the force on the cart, which is less
than the weight of the hanging mass, and the inertial mass of the hanging
mass is irrelevant. (Although a graph of the force measured by the sensor
versus the weight of the hanging mass would be instructive.)

On Wed, 9 Dec 1998, Norman Lohstreter wrote:

I had a parent question the procedure for verifying Newton's second
law. The procedure involves a cart with added mass and a force probe
which is connected by a massless cord that passes over a
massless/frictionless pulley to a smaller hanging mass that provides the
force. The standard procedure (from several lab manuals) says to take
mass from the cart and transfer it to the hanging mass position in order
to keep the mass of the system constant. This seemed to make sense to
me until the conversation with the parent.

His position was, the only mass that was necessary to keep constant was
the mass on the cart and if you transfered the mass from the cart to the
hanging position, you were changing both the accelerating mass and the
applied force and that there was no way to determine the results you
wanted because there were too many variables.

Since the force is being measured by the force probe connected to the
cart/mass component, is it still necessary to keep the mass of the
system constant? We now are able to measure the accelerating force
acting on the cart itself. The force acting on the hanging weight is
different from the tension in the cord because the hanging weight is
accelerating too. There is much similarity to the attwood machine here.

Any assistance is greatly appreciated.

Pete Lohstreter
NGHS