A few articles we ran across this week, two new and one old, have had us thinking about that ever-popular topic for the first week of the semester: Galileo, and the forces acting on objects in free fall.

 One of the basic concepts students struggle with in the early stages of introductory kinematics is the concept of free fall, and how different objects behave when falling. This question takes us back to the classic experiment Galileo may or may not have actually carried out, dropping objects of different masses from the top of a tower and observing that, barring drag, they fall in unison, regardless of their respective masses. Here at the Lecture-Demonstration Facility, we have a variety of demonstrations to help illustrate this concept, many of them quite popular in introductory classes... both because they are helpful illustrations of this important physics concept, and because falling objects make a satisfying bang. If you are teaching this topic here soon, be sure to explore sections C2 and C4 of the demonstrations index to see what we have to offer.

Leaning tower of Pisa; Galileo probably did not actually drop things from here. image courtesy

This was brought to mind recently when we came across an article from 2013 about efforts to rescue the endangered Leaning Tower of Pisa. Despite the legend, Galileo probably did not actually drop balls from the top of this tower; it does, however, make an excellent illustration for discussing the problem, and is popular in many textbooks for this reason regardless of historical relevance. The effort to save the tower from finally falling over entirely does itself lead to some interesting physics questions, discussed in the article, and could be interesting to students as an opportunity to talk about issues of force and torque, equilibrium and the center of mass. To explore other aspects of this problem in class, check out demonstrations B1-02 and B1-03.

In a recent paper in the European Journal of Physics Education, Balukovic & Slisko explore some of the potential causes of student confusion around weightlessness in free fall, and ways to address them. They recommend using multiple demonstrations and problem solving to help students engage in active learning around the topic. They also have suggestions on thinking about how we use language itself to talk about physical problems to improve clarity and understanding.

Leaving the Renaissance largely behind, this classic image of Galileo dropping his spheres is cited again in an article posted last week in Physics Today. Discussing recent research by Hebestreit, Novotny, et al., they report on the latest experiments in using optically-trapped nanoparticles as tiny force meters. When the optical trap is turned off, the nanoparticle “falls” or responds to other outside forces. By rapidly turning the trap off and on, they can measure the acceleration of the briefly free-falling particle to a high degree of precision, and can thus potentiallly use it as a measurement tool.

News items like this can be very useful in class to promote student engagement. Helping students see how the basic concepts we're teaching can be tied in to both cutting-edge research and real-world problems helps them both understand the concepts and better value what they learn.


Balukovic, J., & Slisko, J. (2018). Teaching and Learning the Concept of Weightlessness: An Additional Look at Physics Textbooks. European Journal of Physics Education9(1), 1-14. DOI:

Hebestreit, E., Frimmer, M., Reimann, R., & Novotny, L. (2018). Sensing Static Forces with Free-Falling Nanoparticles. Physical Review Letters121(6), 063602. DOI: 10.1103/PhysRevLett.121.063602

Miller, J. (2018) Free-falling nanoparticles help to detect tiny forces. Physics Today.DOI:10.1063/PT.6.1.20180823a

Watt, S. (2013). Propping up the wall: How to rescue a leaning tower. Science in School, 26.