Gravitación

[Spanish version coming soon]

In this episode, the characters are discussing about adapting the tale "Micromegas" from Voltaire to theatre, as they are practising in a climbing room. Their discussion lead them to wonder about the presence of gravity in Space, considering the state of weightlessness of an astronaut. They discover first that, as the free fall speed doesn't depend on the mass, from the point of view of somebody falling, other objects falling at the same time appear as floating weightless. Motion or not motion then depend on the point of view. They realise that from the Earth's point of you, a weigtless astronaut in a space station is in orbital motion. Also, the astronaut motion is the same as the station, even though their mass is very different, exactly like free fall. The characters finally use the historical "Newton's canon" thought experiment for the end of their play, as way to understand to common points between free fall and orbital motion : both can be link continuously depending the horizontal speed.

With this episode and associated teaching material, students are expected to understand some first qualitative arguments for the extrapolation of the gravity force to Space : if we interpret free fall on Earth with gravity force, and that astronaut orbital motion is "the same kind of motion" than free fall, then we should interpret orbital motion with gravity too. So their should be gravity in Space !

"There is no gravity in Space" is very common students idea, according to Physics Education Research litterature (Kavanagh & Sneider, 2006). It can be linked to other misconceptions as :
- Objects in orbit have no weight, so gravity does not affect them (=> so there is no gravity in Space)
- Gravity requires the presence of air (=> then there is no gravity in Space)
- The force of gravity decreases very quickly as altitude increases (=> until there is no gravity in Space)

These ideas can be linked to the images of astronauts in weightlessness, which are widespread in the media. Since the term gravity is associated in everyday language with the phenomenon of falling, the fact that astronauts do not fall but 'float' seems to imply that there is no gravity in Space, according to a common reasoning that can be summarised as follows: no fall, no gravity.

The invalidity of this logic, from the point of view of physics, lies in the fact that the notion of the force of gravity - or gravitational force - makes it possible to account not only for vertical or parabolic fall, but also for orbital movement in space. Astronauts in weightlessness in their space station, seen from the centre of the Earth, do indeed move in a circular motion around the Earth.

The objectif of the episode is to understand first arguments to convince oneself that there is gravity in Space, even though it's not intuitive at first sight.

Going beyond the initial logic of "no fall, no gravity" (for weightless astronaut) means understanding how the notion of gravity can be extended to orbital motion, which is visually very different from falling. The teaching challenge is therefore to highlight the links between free fall and orbital motion.

The first link is made by considering free fall from the point of view of the falling body: it perceives its situation as a state of weightlessness, just as an astronaut orbiting in space would, from its own point of view. A second point in common (which is another way of looking at the first) can be highlighted by considering the following fact: the weightlessness of an astronaut inside (or outside) a space station implies that the astronaut has the same trajectory and the same speed as the space station, even though their masses are very different. This fact can be compared with the free fall of bodies on Earth, where we know that the speed does not depend on the mass (when the influence of the air is negligible). This consideration constitutes the major specificity of the conceptual approach. Even though it is an accessible information when we focus on weigtlessness, the relation of this observation with mass independance of free fall is almost never highlighted.

After highlighting these common points in Chapter 2, Chapter 3 presents the historical diagram of Newton's 'canon', in which the different trajectories of an object's fall are considered according to the speed given at the start. This shows the continuity between free fall and orbital motion, and helps us to understand the links between them. Vertical fall, parabolic fall and orbital motion can then be seen as three cases of the same type of motion, namely gravity-related motion. Consequently, contrary to the a priori idea taken as a starting point: gravity is indeed present in space. It is to extend its field of validity that we use the term "gravitation".

Historical material

In this episode, the main historical element used is the "Newon's cannon" proposed by Newton in "De mundi Systemate" in 1685 (published in 1728). This thought experiment represents different trajectories for the fall of a cannonball depending on the velocity given.

Epistemological perspective

After more than 2000 years between of distinction between the terrestrial and celestial worlds, Newton established a strong physicial link : the phenomena of free fall on Earth and the orbital motion in Space can be understood as the same kind of motion, interpreted by the same force. This force generalises the notion of weight to the stars: the centripetal force associated with their revolution is ultimately their weight. Gravity is universal.

The world system can then be described in a single way, with the same force explaining both terrestrial and celestial movements. By unifying the Earth and the Heavens, Newton gave birth to the concept of Universe.

After Newton, all the fundamental laws of physics were considered (implicitly) to be "universal", or sought as such. But the starting point for such an ambition came from physics itself: it was necessary to find a first law that was valid everywhere in order to assume that all the others should also be valid.

Prerequisite : consider mass independance of free fall in advance

An essential prerequisite concerns the phenomenon of free fall. Although this is a very common phenomenon, the first classroom sessions using the comic showed that many high school students can still be astonished by the fact that an object's speed of fall is independent of its mass, when the influence of air is negligible. Their focus on this point has diverted their attention from the information that comes later in the story, aimed at linking the falling movement and the orbital movement. It is therefore strongly recommended that this observation is recalled before the session, so that the students can focus more on chapter 2.

Possible resource:

To begin with : what's the scientific problem ?

Following an initial individual reading of the comic by students, it may be useful to begin by explaining the physical problem that emerges in the first chapter: "Is gravitation present in space or not? This can be done by questioning students about the position of the various characters on this issue, and their arguments.

Collective reading with questions on chapter 2

After an individual reading, it is possible to propose a collective reading with a videoprojection of the comic, with the rythm controlled by the teacher. He/She can then ask some questions about some key moments. For instance the folowing :

The aim of this question is to make the link between the independance of the mass in free fall and weigtlessness.
Anwser : the same, because free fall speed doesn't depend on the mass

This question is for testing the understanding of the point of view dependance of motion description.
Answer : he can be moving, from another point of view

Asking about the mass aims at raising awareness that there is a big difference between the two (by guessing or with the answer). Focusing on this feature may help to realise that the motion is the same, although the mass are very different, as for gravity.
Answer : more than 400 tons !

Answer : Free fall : same motion whatever the mass !

From physics to the comic

Once you have interpreted the scenes, you can also take the opposite approach: ask students to illustrate general physics statements using specific images from the comic. It is possible also to make them link explicitly the common points between falling and orbital motion. Here is an exemple of exercise (editable version below).

Document A

Document B

Editable teaching material

3 mo

On the link above you can get all to documents above in a ppt format, so you can edit them yourselves.

Proposition 1 : Two classes teaching sequence

- First class (1h)

Individual reading (5 minutes)
Following an initial individual reading of the comic by the pupils, it may be useful to begin by explaining the physical problem that emerges in the first chapter: *"Is gravitation present in space or not?* This can be done by questioning the position of the various characters on this issue, and their arguments.

Questions on chapter 2 (20 minutes)
An important stage in class work on the comic strip is to draw the pupils' attention to the key scenes for which reflection is needed to grasp the physical ideas that will provide an answer to the initial problem. These are the two changes of point of view staged in chapter 2: the fall seen "from the inside" (p.8), and the state of weightlessness seen "from the outside" (p.46). An example of questioning is given in document A below. Once you have interpreted the scenes, you can also take the opposite approach: illustrate general physics statements using specific images from the comic strip. By formulating these statements, you can also clearly highlight the similarities between falling and orbital motion. This is what is suggested in document B below, for example.

Work on documents A and B (20 minutes)
Once these points in common have been made clear, the question of how to explain them can be asked: how can we understand that these movements, which seem so different, share the same characteristics? The "Newtonian canon" thought experiment referred to in Chapter 3 can then be used to understand the continuity between these different movements, thereby justifying the points they have in common. The book extract in Document A above offers an original way of interpreting this diagram. At this stage, the link between the different elements may not yet be very clear to the pupils. An explicit link may be necessary. Document B provides a short summary of this. In particular, it clarifies the difference between the terms "gravity" and "gravitation", and makes it clear that orbital motion can be associated with gravitation. There is then just one more step for the students to take to deduce that gravitation is indeed present in space, which answers the initial problem.

- Second class (1h)

The second class is about working on reconstructing the reasoning that leads to answer the initial problem. After reading again the comic episode, they are given the following instruction: "Many people think that there is no gravity in space, having in mind astronauts who 'float'. Using the comic strip and the work from the previous class as a basis, write a short text aimed at convincing people that, despite the weightlessness of astronauts, gravitaty does exist in space". As this task might be quite difficult for students, different levels of intermediate help may be needed.