Re: Newton's 2nd law Lab

[John Gastineau <john@GASTINEAU.ORG>]

> 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.

While this procedure should work since you are _measuring_ the tension force
applied to your varying mass (ugh!), not inferring the force, (I assume you're
measuring the acceleration of the system indirectly by using a photogate or a
motion detector) the experiment is not general and is messier than necessary.
You're only using constant forces, while Newton's 2nd is also about
time-varying forces.

A much cleaner experiment is possible using a force sensor and an
accelerometer. Attach both devices to a cart; move the cart by hand using only
the force sensor. Record both the force vs. time and acceleration vs. time
data, then plot force vs. acceleration. The points should be on a line with
slope m, where m is the total mass of the cart, accelerometer and force
sensor. Now change mass of the cart. New slope.

This experiment is much, much cleaner than the old half-Atwood method for
doing Newton II. The half-Atwood approach done with MBL is just a computerized
version of a non-computer method.

You can also do this experiment with a motion detector and force sensor, but
the data quality suffers considerably since you must work with a second
derivative to get the acceleration data, while the accelerometer gives it to
you directly.

I can point you to more details if needed.

JEG
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John E. Gastineau      john@gastineau.org               KC8IEW
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