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Learning to Think about Gravity: Newtons's Theory

Esther Zirbel, Tufts University
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Leading questions: How and why do the apples fall to the ground? Why does the Moon not fall from the sky? What is gravity?

In the past 20 years the concepts of force and motion and Newton's three laws have been discussed widely in the science education literature, however, the concept of gravity has not yet been discussed extensively. We will address the concept of gravity using a historical constructivist approach: We will review Aristotle's views on gravity and compare and contract that to Newton's views. We will introduce the possibility that gravity is a force and then discuss the effects of this force on objects in motion. We will also discuss how the Cannon Ball thought experiment proves that the Newton's Law of Gravity is universal and contrary to Aristotle's concepts is applied to heavenly as well as earthly objects.

Learning Goals

1) Students will learn the Newtonian concept of Gravity and how that Theory was developed
2) Students will learn to respect their own theories as well as those of others. They will learn that science is not absolute and that Aristotle's concept of gravity was believed by humanity for 2000 years.
3) They will make the transition from Aristotelian to Newtonian thinking.
4) They will learn how scientific methodology is employed to describe motion accurately. They will also learn how to use thought experiments in addition to making measurements.
5) They will learn to differentiate between an empirical model that fits the data and a universal theory.

Context for Use

Comparing and contrasting different theories of gravity and leading the students to transition from Aristotelian to Newtonian thinking. This activity is particularly useful to teach how theories are constructed and how scientific methodology can lead to a deeper understanding of the world around us. It can be used as an introductory class on the nature of science.

This exercise works very well with a class size of roughly ten students, but has also been successful if applied to larger classes of 50 students. This activity is desiged for an introductory lab/lecture, but it can be adopted in other settings.

It is an interactive lecture that includes Socratic dialogue, in class demos, and thought experiments.

It can be done in one hour. However, two hours work better especially if the "rolling-cans-down-an-incline" activity is also performed. For that three identical cans are needed, one empty one, one filled with beans, and one with a clear broth.

There are no pre-requsite skills or concepts that students should have mastered before encountering this activity.

Description and Teaching Materials

The power point presentation of the class is attached. (PowerPoint 4.8MB Jul24 07)

The historical-constructivist method of teaching will be employed.

Student's ideas will be elicited.

Followed by an interactive lecture with two experiments (described below) and several discussions.

The final activity consists of looking back at the original pre-instructional ideas and "correcting" them.

Optional: A discussion on the Misconceptions of students can be included. A list of these misconceptions is attached. -- draft -- file will be updated later (PowerPoint 3.2MB Jul24 07).

Experiment 1:
Drop various objects, let students predict how they will fall and then do experiment. Do experiment "qualitatively". Use block of various sized, pieces of paper and crumbled together paper. If the apparatus to make a vacuum exists, do the experiment of dropping a coin and a feather.

Explain Galileo's original experiment of how he determined s=1/2at-sq by rolling balls down an incline that was carved out at equal intervals.

Experiment 2:
The do the experiment of rolling three cans of identical shape down an incline. One can will be empty, the other filled with a clear liquid, the other with solid food. Let students make predictions of which can will roll fastest. Also let them explain their reasoning for their predictions. Let this be some type of game and/or competition. Finally do the experiment, congratulate the winners, and discuss with them all effects and variables that determine the experiment. Discuss concept of inertia. Discuss experimental variables and idealized situations. Finally have a discussion on the purpose of that activity -- what are the students supposed to learn about scientific methodology from doing that activity?

Teaching Notes and Tips

Students should already be familiar with Newton's three laws

Always elicit student's ideas

Don't correct student ideas, but let them discuss their ideas among themselves and reach a consensus.

Let students come up with conclusions themselves.

Power point slides are meant as a guidance only, or to be presented after the students have come up with their conclusions.

Final discussion is important.

Students have many misconceptions about motion and this has been research extensively in the science education literature. Research of Newton's Universal Law of Gravity has been reviewed by Kavanagh & Sneider (2007) and the reader is referred to that article about students misconceptions and about methods of teaching Newtonian gravity.


Use the FMCE for Newton's three laws. A Tool to assess Newton's Universal Law of Gravitation is currently being developed by the author.

References and Resources

The instructor is strongly encouraged to read the following two papers by Kavanagh & Sneider (2007) on teaching gravity:

Learning about Gravity I. Free Fall: A Guide for Teachers and Curriculum Developers

Learning about Gravity II. Trajectories and Orbits: A Guide for Teachers and Curriculum Developers