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AN INTRODUCTION TO NEWTONIAN
MECHANICS by Edward Kluk Dickinson State University, Dickinson ND |
A historical overview
An "experiment" on frictionless surface and data analysis
A single push of an object resting on a rigid horizontal plane makes this object moving along
a straight line. But after a while the object will stop. This simple observation is known for human beings for many
thousands years. In 4th century BC Greek philosopher and scientist
Aristotle postulated that keeping a body in motion
with a constant speed demands a constant push. Or we may say, application of a constant force. But we have to admitt a
vagueness of idea of force in this kind of statement. It does not tell us how to measure a force, then it does not have
practically any scientific value. It took two thousands years and
Isaac Newton to refine Aristotle's postulate and create fundamental theory of motion. Here we will make a first
step in this direction.
Actually it is not difficult to perform a following experiment. Take a hockey puck, set it flat on a well
leveled big sheet of sand paper and give it a single slight push. The puck will not move very far. Replace the sand paper by
concrete surface, a board covered with formica, and finally a smooth ice surface. It is clear that the same push will make the puck
to travel longer distance as we are changing the surfaces. Probably everybody would say that such results are obvious because
in each consecutive case we deal with less friction. A point here is to imagine yourself what would happen if there were no
friction at all. Well, it looks like the puck should move with a constant speed across such frictionless surface. Then Aristotle was
not quite right because if there is no friction, no force is needed to keep a body in motion.
The last paragraph presents an example of abstraction method applied in science. Without having
possibility to experiment with a frictionless motion we were able to deduce that keeping body in motion does not require any
force, unless a friction, or possibly other forces trying to stop the body are present. Therefore bodies by their nature are
trying to preserve (conserve) a status of their motion. Understanding how the simple experiments with hockey puck lead to
such conclusion is very important as an introduction to Newton's laws of motion. Predesessors of Newton were not able to realize
that if a force is needed to keep a body in motion this force is used only to overcome friction forces trying to stop the body.
This experiment is designed to help you understand relations between an abstract frictionless
motion and its mathematical model (description). Select an initial speed, start the motion and collect data representing covered
distance versus time. To do so, you have to start the motion with selected initial speed several times. Remember, you cannot
stop this motion. When it stops, reset it to its original position and start again. You may collect data for two or three different initial
speeds.
Now graph your data and convince yourself that the results can be described with help of the
relation
Math helps to reach more conclusions
Math wizards can see right away from the distance - time relation that speeds for investigated
motions are constant and equal to vo of each motion. If you are not sure about it use the formula for
an average speed vav which we have introduced in the former experiment
Evaluation
If at this point you do understand:
Last update: Jan 10, 1997 | E - mail to Edward Kluk |
Copyright (c) 1996 Edward Kluk |