Northern Illinois University
Physics 150
Lab 4 Recitation
Atwood's Machine
Whether or not Isaac Newton was inspired by a falling apple, his genius resulted in several
laws of physics. By equating the force on an object with the product of the object's mass and its
acceleration summarized by the following equation:
F = ma
Newton specified what became known as his Second Law. In words, an object in motion tends to
stay in motion unless acted upon by a net outside force. Similarly, an object at rest tends to stay
at rest unless acted upon.
Another law of science credited to Newton, called his Third Law is the concept of force pairs.
You may be more with the expression that says that for every action, there is an equal and
opposite reaction. The action/reaction is considered a force pair. When you lean against a wall,
you are exerting a force against the wall. At the same time, the wall pushes back against you with
exactly the same amount of force. This is also true for objects in motion. If you kick a rock,
exerting a force that accelerates the rock, the rock also exerts an equal and opposite force on your
foot. As you know, the bigger the rock, the bigger the force it takes to accelerate it, and the more
your foot hurts!
A very helpful technique for analyzing forces (and the motion that may result) is to draw free
body diagrams. Some practice of this technique will serve you well in almost any physics
problem involving force and motion. The basic steps are:
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Isolate each component of the system by drawing it alone.
Working with only one component at a time, identify (and draw as an arrow)
every force acting upon the object. The direction that the force is acting is
indicated by the direction of the arrow and the (relative) magnitude of the force is
indicated by the length of the arrow.
Assign a set of coordinate axes to each diagram, selecting an orientation that
minimizes the trigonometry necessary to evaluate the forces and motion.
Sum forces along your axes and set them equal to the object's mass times its
acceleration along each axis. Many problems will be simplified by noting the
absence of acceleration along a particular axis.