# Physics Unit 5 - Constant Net Force Particle Model

This unit is about ascribing causes to the motion that is described by the constant acceleration model

You should be reading Chapters 4 & 5 in the textbook as we study this unit.

The modified Atwood's machine is a cart towed across a tabletop by a hanging mass.  We observed the cart's motion, and many students noticed that it visibly accelerated.  Recall from the dry ice block that constant force begets constant acceleration.  You suggested that changing the hanging mass or changing the mass of the cart would affect the acceleration. We performed two experiments to test this.

In our analysis we ignored the mass of the string, inertia of the pulley, and we tilted the track to minimize or eliminate the effect of friction in the wheel bearings and pulley.  When given a brief push, the cart rolled with a relatively constant velocity, evidence that friction did not have a significant effect

We did two experiments.  In experiment 1, we changed the hanging mass without changing anything else.  This meant that when we took mass off of the hanging mass we put it on the cart, or vice versa.  Since we changed the hanging mass, that changed the force of gravity on the hanging mass, which changed the towing force on the cart.  We plotted acceleration vs. towing force.

The equation for this graph is

Accelerationsystem=1/masssystem*Towing Force  OR Newton's 2nd Law

In the experiment 2 of the paradigm lab, we investigated what happens if you change mass of the cart, but keep everything else the same.

The equation for this graph is

Accelsystem=Net force/mass of cart

Since we noted that the coefficient was slightly less than the towing force, it made sense to call it net force.  Some friction was present, and the mass/inertia of the string and pulley acted to reduce the value of the coefficient.  In an ideal experiment, it would have been exactly equal to the towing force.

Short Summary: the acceleration of the system is proportional to the gravitational force on the hanging mass and inversely proportional to the system mass (cart and hanging mass).

Random Notes