Experiences

Abolic motion. Measure of the throw speed.

Gomboli Sandro

Italy

Vocational School

Physics

Laboratory

Direct

The mathematical language is closely related to a natural phenomenon, not always this bond is actually understood by pupils.
The value of this experience, made with materials available at low cost, is that you can almost materializing the close link between physical phenomena and mathematical language and to understand the power of scientific inquiry.
The parabolic motion, consisting of a rectilinear uniform motion and uniformly accelerated motion is solved by a system of two equations. A metal ball is launched from various height with the same speed initially horizontal. Detecting the distance of a fall from various heights can reconstruct the parabolic trajectory.
In this study purely kinematic making some legitimate simplifications: the ball thrown by a launching device is approximated to a material point, the friction forces are considered negligible.
This experiment, to be carried out with the second year students, consists in the throw of a steel sphere from various heights with a horizontal starting trajectory disclosing the fall point on a sheet of paper on which a traceable sheet of paper has been set.
The launch device consists in a tube through which the ball is let free to fall allowing the repetition of the throw with the same starting cinematic features.
The starting point at the exit of the device is set on the vertical line coinciding with the edge of the sheet of paper on the table which is in front of it using a plumb line.
Drop heights are progressively varied, measured with a measuring tape, and three drops of the sphere are done for each of them highlighting with a progressive number the mark they leave on the paper.
Once a complete series of throws is carried out, the distances of the marks from the edge of the sheet are measured.
For each test you statistically analyze the three values of the fall distances obtaining the average values and their uncertainty (usually due to casual and not negligible errors).
The obtained data (heights and distances of fall) are then put in a graph that shows the distance of fall in function of the height obtaining the parabolic trajectory of the motion.
In this experiment, the tests number is considerable (not less than six) in order to allow the traceability of the parabola.
The initial velocity of the constant throw is obtained using the hourly equations typical of the motions constituting the parabolic motion (uniform rectilinear motion and uniformly accelerated motion) also evaluating the uncertainty through the analysis of the uncertainties propagation.