It shows which equipment is required, the precautions that must be taken and the
methodologies that must be used.
From a didactic point of view we ask the experiment to be:
That can be obtained in different ways:
rectilinear motions
This is a short guide to the experimentations on rectilinear motions.
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Any experimental study requires a careful choice of the instrument to be used.
In fact the phenomenon to study must be characterised as precisely as possible by removing or
reducing to their minimum all the causes of perturbation that can affect its development.
Besides, the collection of the experimental data must be easy and ascertained and also
the processing must be fast and simple.
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The main cause of perturbation, in the study of a motion, is of course friction:
a good experimental device must therefore eliminate or reduce the friction substantially.
The residual friction can be compensated by a light inclination of the track on which the trolley moves.
To obtain a guaranteed rectilinear motion we can use a rail on which the trolley moves.
The substantial flaw is having rotating masses that can create problems (e.g. when we want to enhance the kinetic energy).
When subliming, the dry ice produces a gas of carbonate anhydride that escapes through a hole in the centre of the puck. We have, between the puck and the surface of support, (very smooth, generally a plate of crystal) an air cushion that holds the puck lightly raised over the plane of support.
Practically we obtain a motion without friction.
For a didactic use, the problem arises because of the necessity of producing the dry ice for each experiment.
The air track allows to obtain easily un rectilinear motion and it can be inclined by an angle large enough (without the glider touching it) to allow the study of the motion on a inclined plane.
The systems to collect the experimental data (space and time) can be:
Anyway a multiple chronometer can be used as that showed in the figure, connected
as in the schema.
The time intervals are measured by the computer clock
Programs to process collected data are normally enclosed in the interface.
Flaws: the method didactically presents some counter-indication because of the
automatic data collection and process, except if we use it as comparison with the manual work
of pupils.
The space covered is measured on the strip and the time is obtained as the
sum of the elementary times between two contiguous points.
Flaws: the period is always fixed (normally 1/100 s, depending on the frequency of
the electric current); the strip dragged by the mobile creates practical problems when
effecting the measure and can easily perturb the motion of the mobile.
The detection of the data occurs, as with the time-recorder, by directly measuring
the space between a point and the other and by summing the elementary time intervals.
Advantages of the spark generator over the time-recorder:
A stroboflash sends periodically a flash that exposes on the film the position taken
in these moments by a flag fixed on the glider.
If a metric scale is fixed on the track of the glider, on the film we can read the
position taken by the glider moment by moment, while the times are obtained, also here, as the
sum of as many periods as the number of the positions reached.
It is necessary to have an instant camera; it is necessary to operate in the dark
(even if the experimental device can be modified by illuminating the glider constantly and
disposing a stroboscope in front of the objective of the camera).
Flaws: the experimental device and the execution of the experiment are rather complex.
The camera can be directly collectable to the computer and it is provided with
software for the data processing.
The experiment does not present any problem of illumination.
The process of the experimental data consists in:
The system based on arranged spaces consists in settting the position in
correspondence of which we will measure the times. We use a chronometer that is started
and halted by the mobile with gates through a mechanical or an electrical contact or
photocells.
Flaws: for this study the knowledge of only one interval of space with the
relative time is not enough; for which it is necessary to dispose of different chronometers
linked to different gates ( with great experimental complication) or to repeat the motion many
times, moving the gate each time, with very doubtful experimental and didactic results.
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The system based on arranged times consists in setting the intervals of time in
correspondence of which we will measure the spaces.
The time-recorder is made of a vibrator (the first time-recorders were constituted by
a modified electric bell) to which a point is fixed that leaves signs on a paper strip dragged
by the mobile at constant intervals of time.
The spark generator periodically produces impulses of high tension that arouse
sparks between a wire linked to the mobile and a spark paper placed on the track. The spark
paper blackens in correspondence of each spark.
- the spark generator allows choosing the value of the interval of time, within quite
a large range.
- the spark paper does not disturb the motion at all, as it is not dragged.
The glider moves in the dark in front of a camera with the shutter constantly open.
We apply to the body in motion a self-adhesive strip that reflects the luminous
impulses emitted by the LEDs that the camera uses as reference to the position of the bodies.
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Whatever method is adopted, at the end of the experiment we have the values of
space and of time and the relative experimental errors depending on the instruments used.
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