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Newton's 2nd law Lab



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