Lifelong Learning Programme

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Title of the Experience
The principle of inertia
Name of the teacher
Renzi Alessandra
Country where it took place
School typology
High Secondary School
Thematic Area
Experience typology
Teaching in class, Laboratory
Type of contact
Description of the Experience
I would like to describe a laboratorial experiment related to the principle of inertia.
To implement this laboratorial experience it is not necessary to have a physics lab, but you just need to have a bit of practicality.
Material to be used: a hula hoop, a sheet of carbon paper, cardboard and a small iron sphere.
Preparatory phase: you have to cut the hula hoop in any part of the border, the hole has to be about 0.5 cm wide (actually it depends on the size of the sphere used).
The circle in put on a horizontal surface on the cardboard, as shown in the figure at the bottom of the text.
We ask students the following question: " if I throw the ball inside the circle, allowing it to rotate using the border of the hula hoop, do you think the ball will come out or will it continue to spin in the circle (assuming of course that you have given enough speed to turn)?”
"Do you think the outcome depends on the velocity imparted on a ball or not?”
The debate starts. The majority of students, you will see, will support the hypothesis that if you throw the ball with a high speed, it will continue to turn.
Only a few students would answer that the ball always goes out.
The correct answer is that the ball always comes out. This is because of, to have a circular motion, the ball needs a certain centripetal force. But this is provided by the rheonomic constraint of the hula hoop and, in the exact point where the hula hoop is cut, this force stops to exist. Thus, according the principle of inertia, the ball will move in uniform rectilinear motion, continuing along the direction of the tangent of the cut point of the circle.
In this simple experiment, there is a lot of physics: the rheonomic constraint provides the centripetal force, the principle of inertia.

Comments on this Teachers Experience

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Date: 2014.11.10

Posted by Maria Fabianova (Slovakia)

I very appreciated this experience. It is very nice and practical visualization of the inertia movement and this experience is easy to use in class. The next very good reason to use this experimwnt is that is very atractive for students with using of relative cheap materials. I use similar experiment in my class but this is better for understanding this motion and for explaining that the circular motion needs a force. I will use this experience with my students in future.

Date: 2014.10.16

Posted by Milena Gosheva (Bulgaria)

I think this is very good visualization of the inertia movement. The experience is easy and always applicable in classes in mechanics. Using tracing paper is to record the path of the iron ball and compare the trajectories of the field in the dense ring and cut. I would definitely use this experience with my students.

Date: 2014.09.30

Posted by Sandro Gomboli (Italy)

The experience allows to empirically verify the first principle of dynamics, the so-called Principle of Inertia, using easy-to-find and cheap materials.
But this is not the unique value of this experience.
The qualitative feature of the current experience allows concentrating the attention exclusively on the model object of the study.
Moreover the experience could lead to the introduction to the concept of constraint and of constraining force and to mark the (linear and circular) trajectories of the motions.

Date: 2014.09.30

Posted by Paola Falsini (Italy)

I appreciated a lot this experience.
A colleague of mine suggested to provide our laboratory with this tool and I always use it when I have to explain that the circular motion needs a force.
According to the description of the experience, I think it is very important to ask questions to the students before performing the experiment.
I admit I forgot to ask to the students if they think that the sphere motion, when the track is interrupted, depends on to the initial force.
I will make this question in the future for sure.
I confirm that the students observed the sphere going beyond the track with a linear motion. This observation is a valid reference model for every curvilinear motion. For example, it is helpful to understand that the motion along an elliptical orbit needs a force.
Every time that some doubt rises on similar situation, we can say: do you remember the interrupted circular track? And thus the students have their reliable reference point, like an experiential data should be.
In order to reinforce the result of this experiment , I read to the students an extract from Principia Philosophiæ (Principles of Philosophy) by René Descartes: “The second law of the nature is that each moving body tends to continue its motion along a linear line… and not in a circular way”. This Descartes’ clarification makes us aware about how much the idea of the circular motion as a natural motion (coming from the millennial observation of the celestial bodies) is hard-established. The Grecians thought like our students, like the common sense suggests to think, that is the circular motion is able to preserve itself.
These considerations help us to understand how far the Physics is from the common sense and how much attention have to be paid building the fundamental ideas of the dynamics, without ignoring the naïve concepts of the students.


Dissemination Seminar in Florence

31 October 2015 The seminar has been held in ITIS “A. Meucci” one of the schools involved within Goerudio project activities. Its main aim was to promote the results of the project toward a broad sample of stakeholders even overcrossing the number of people directly involved in the production of project outcomes. This purpose has been totally achieved especially thanks to the participation of students and teachers coming from different schools or from other classes instead of those ones directly involved within the project activities.