Section 5–1 Motion

(Motion / Galileo’s clock / Galileo’s inclined track experiment)

This chapter was delivered by Matthew Sands because Feynman was out of town. In this section, the three interesting points discussed are the concept of motion, Galileo’s clock, and Galileo’s inclined track experiment. (Feynman elaborates on the concept of motion in chapter 8.)

1. Motion:

“… the study of motion had been a philosophical one based on arguments that could be thought up in one’s head (Feynman et al., 1963, section 5.1 Motion).”

According to Dr. Sands, the development of the physical sciences to their present form has depended to a large extent on the emphasis which has been placed on the making of quantitative observations. Based on these observations, physicists can deduce important quantitative relationships, which are the heart of physics. On the other hand, the study of motion had been mainly philosophical based on arguments or thought experiments. Most arguments presented by Aristotle and other Greek philosophers were accepted as “proofs.” Thus, Galileo was skeptical of Aristotle’s theory of motion and carried out experiments on motions.

It is controversial to identify Galileo as the first physicist. For example, one may prefer Aristotle because he is the first person that writes a book titled Physics and proposes that the motion of an object is influenced by a motive force and resistance of medium. It is also debatable whether Galileo should be recognized as the “first experimental physicist.” For instance, Giuseppe Moletti reported in 1576 that objects made of the same material but having different weights reach the ground at the same time (Martínez, 2011). Currently, it is commonly agreed among historians that Galileo conceptualizes a thought experiment of dropping two spheres instead. Galileo’s dropping of two spheres of different masses from the Leaning Tower of Pisa could be an apocryphal story (Martínez, 2011).

Note: In chapter 8, Feynman discusses Zeno’s paradox of motion and defines motion as “the apparent change in its position with time.” In addition, he briefly explains some difficulties of defining space and time precisely from the perspectives of the theory of relativity and quantum mechanics. Although the concepts of motion, space, and time involve deep philosophical questions, Feynman suggests that we should avoid the paralysis of thought in defining everything more precisely.

2. Galileo’s clock:

“…he later devised more satisfactory clocks (though not like the ones we know), Galileo’s first experiments on motion were done by using his pulse to count off equal intervals of time (Feynman et al., 1963, section 5.1 Motion).”

Dr. Sands opines that Galileo’s first experiments on motion were done by using his pulse to measure time. However, Galileo’s measurement of a pendulum’s period by using his pulse while he was watching a lamp swinging in Pisa cathedral may be another apocryphal story (Glennie & Thrift, 2009). More important, the use of one’s pulse rate to measure time or a pendulum’s period in an experiment would introduce inaccuracy or subjectivity. Thus, the use of pulse to compare time is just a short-term improvisation. On the contrary, Santorius (1561–1636) was using a pendulum (or pulsilogium) to measure the pulse rate of patients. This invention was based on Galileo’s observation that a pendulum’s period is inversely proportional to the square root of its length.

Note: The pulsilogium (or pulsimeter) was composed of a heavy lead bob, a silk cord and a ruler (Sanctorius, 1631). The period of this pendulum bob can be adjusted by varying the length of silk cord from one end of the ruler such that it synchronizes with a patient’s pulse beat. On the scale of the ruler, we can read the frequency of the patient’s pulse beat.

3. Galileo’s inclined track experiment:

“…He allowed a ball to roll down an inclined trough and observed the motion. He did not, however, just look; he measured how far the ball went in how long a time (Feynman et al., 1963, section 5.1 Motion).”

Dr. Sands explains that Galileo’s first experiments on motion were carried out by using Galileo’s pulse to count off equal intervals of time. However, in his book Dialogues Concerning Two New Sciences, Galileo describes the use of a water clock to carry out his experiments on motions. In his words, Galileo explains that “[f]or the measurement of time, we employed a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent (Galilei, 1638/1914, p. 179).” It is possible that Galileo used a water clock and music (or a song) to measure time during experiments (Drake, 1975).

Feynman might attack Galileo’s deduction of how an object would move in a free fall from the inclined plane experiments. In Surely, You are joking, Mr Feynman!, he criticizes a physics textbook and says that “[t]here are no experimental results mentioned anywhere in this book, except in one place where there is a ball, rolling down an inclined plane, in which it says how far the ball got after one second, two seconds, three seconds, and so on. The numbers have ‘errors’ in them--that is, if you look at them, you think you're looking at experimental results, because the numbers are a little above, or a little below, the theoretical values. The book even talks about having to correct the experimental errors--very fine. The trouble is, when you calculate the value of the acceleration constant from these values, you get the right answer. But a ball rolling down an inclined plane, if it is actually done, has an inertia to get it to turn, and will, if you do the experiment, produce five-sevenths of the right answer, because of the extra energy needed to go into the rotation of the ball. Therefore this single example of experimental ‘results’ is obtained from a fake experiment (Feynman, 1997, p. 217).”

Questions for discussion:

1. Who would you identify as the first physicist?

2. Which clocks devised by Galileo are accurate for an experiment?

3. How would you perform Galileo’s inclined track experiment?

The moral of the lesson: it is important to perform experiments to study the motion of an object quantitatively.

References:

1. Drake, S. (1975). The Role of Music in Galileo's Experiments. Scientific American, 232(6), 98-104.

2. Feynman, R. P. (1997). Surely You’re Joking, Mr. Feynman! : Adventures of a Curious Character. New York: Norton.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. Galilei, G. (1638/1914). Dialogues Concerning Two New Sciences. New York: Dover.

5. Glennie, P., & Thrift, N. (2009). Shaping the Day: A History of Timekeeping in England and Wales 1300-1800. Oxford: Oxford University Press.

6. Martínez, A. A. (2011). Science Secrets: The Truth about Darwin's Finches, Einstein's Wife, and Other Myths. Pittsburgh, Pa.: University of Pittsburgh Press.

7. Sanctorius, S. (1631). Methodi vitandorum errorum omnium qui in arte medica contingunt. Geneva: P. Aubertum.