What is Oscillation by Friction?
...... 
If you put a rod on two wheels, which are driven by two
equal motors with opposite turns, it starts oscillating. 
Explanation
If you move the rod to the left, you put more weight
on the left wheel than on the right one. Therefore the force of friction
on the left is bigger than on the right. The rod is moved to the right.
If the main emphasis is on the middle, the forces neutralize themselves.
The rod is moving beyond the centre line because of inertia, until the
force of friction is bigger on the right. The rod is moving to the left.
 The rod is oscillating.
This arrangement can be set up quickly. You can use any
motor, also those from LEGO. You can use a pencil as a rod.
Simulations
with 'Interactive Physics' top
You can simulate this experiment with the computer program
"Interactive Physics". Use a rectangle and two motors with two circles
on them. (I used the standard settings as much as possible). The following
screen is produced.

Formula of
the Oscillation
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There are sine oscillations. A calculation confirms this:
The linear law of forces is valid. Hence you have an harmonic motion. The
formula for the period is:
T is the period, 2d the distance of the axes of the motors,
f the value of kinetic friction and g the acceleration due to gravity.
It is remarkable that the period is independant of the
mass of the rod.
In practice you have a light increase of the period with
the mass: The motors turn less, if you have weight. If you always regulate
the motor to the same frequency with the help of stroboscope lightning,
the supporting surface becomes warm. f is changing.  You can prove the
dependance T=T(d) to your satisfaction. Experiments with friction are delicate.
You can measure value of kinetic friction directly, if
the motors run in the same direction and if you hold on the rod by a dynamometer.

Bibliography
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Gamow, Cleveland: Physik in unserer Welt 1, München
1963
KarlHeinz Gronemeier, Otto Kranz, Hans Steidel: Fachmethodische
Arbeiten zur Mechanik, Shaker Verlag, Aachen 1999
Comments top
The experiment is worthy of being more wellknown. The
experiment is simple. The mathematics to this experiment are elementary.
This is another example of an harmonic motion, which
belongs to the most famous phenomenons of physics.
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©
2000 Jürgen Köller
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