It is difficult to motivate students to explore force and motion, but the ideas in these units are very powerful and have a fascinating history. They explain the motion of the planets and moons, the swirling of galaxies as well as the arc of a flea's jump, and the vibration of molecules we perceive as temperature. Low-friction experiments much like the ones in these units made Galileo realize that the sun was at the center of the solar system. Almost a century later, Newton invented calculus and the law of Universal Gravitation to prove that Galileo was right. Newton used his proof to make a very public prediction of the return of Haley's comet. His success changed society by making everyone more confident that the world was rational and that everything had an explanation, even if it was not yet known.

A video* or assignment about Galileo, Newton, Copernicus, Brahe and the story of the development of our understanding of the motion of the planets might help motivate students. Some students might find the social and religious angle interesting: the impact of Newton on rationality and the Enlightenment and Galileo's renunciation of his findings. Students with an interest in history might like to try to reproduce Galileo's experiments using rolling balls and ramps. These excursions may appear to waste valuable class time, but they can build motivation while addressing standards on the history, social impact, and conduct of science.

In this set of units, we ask students to observe some rather prosaic ideas about how forces and motions are related. It all seems rather obvious until you look closely. These units use some wonderful tools that help students understand the concept. The Force probe and SmartWheel are very quick and sensitive. They generate computer records that make it easy to see and analyze small, fleeting events. Imagine how excited Galileo would have been to have these tools! It is important to create an atmosphere that encourages careful and critical observation. Having great tools is of no use unless students understand and carefully observe the data that the tools capture.

When students have questions, keep returning them to the data, even if you know the answer. Ask:

• "What did you see?"

• "What does the graph reveal?"

• "What evidence supports that idea?"

• "What experiment would you do to investigate that question?"

(Questions like this are particularly handy if you are unsure of your answers!)

Part of the reason for teaching science is to convey the spirit of science to students. This set of units can do this very clearly. It demonstrates how deep thought about careful observations of some simple motions leads to a broad understanding of the motion of anything. Modern, computer-based instrumentation, along with old-fashioned clear thinking and careful use of words, aids in achieving this understanding. Most science today combines all these features.

*We recommend a series of videotapes called "The Inventors Specials". In this series, use "Galilieo: On the Shoulders of Giants" and "Newton: A Tale of Two Isaacs" available from Amazon. In each, a fictional story of a youngster is entwined with the scientist, giving students a persona with whom to identify while the science is addressed. To dilute the impact of all these "dead white men", you might want to show the video on Marie Curie when you do the energy unit.