Newton’s formulation of his three laws of motion in the Principia (1687) laid the foundations for classical mechanics. His ordering of the laws reflected his purpose of building a deductive theory on the basis of a handful of axioms. But there is no prima facie reason for the laws to be presented in the same order in all contexts. When it comes to teaching them, repeated presentation in generations of textbooks has given Newton’s ordering an air of inevitability. Of course we should teach the first law first and the third law last...right?
Fast forward to the 21st century. A growing body of pedagogical evidence shows that Newton’s laws are not being taught very well in schools. Students arrive at university physics courses with lingering misconceptions and contradictions that stand in the way of a deeper understanding of forces and motion.
One reason for this is that the laws challenge some deep-seated intuitions that we all share. They conflict with common misconceptions about force, mass, weight, acceleration and momentum. So it’s easy enough to understand the laws in isolation, but it’s extremely difficult to apply them consistently to concrete situations. Yet students learn best inductively, starting with concrete experiences and working up to general laws. We should teach the laws in a way that tackles the misconceptions and allows students to build a coherent scientific understanding.
With this in mind, a number of recent papers (see further reading below) have concluded that it’s better to teach the laws “out of order”. Although the recommended orders vary (including 1-3-2, 3-1-2 and 3-2-1), a common theme is that the third law should be introduced before the second.
The third law describes pairs of forces between interacting objects, capturing the fundamental point that forces never occur in isolation. A proper understanding of the third law allows students to identify the types and sizes of the forces acting in a concrete situation. It also challenges the misconception that a force is a property of an object instead of an interaction between two objects. For example, it undermines the misguided idea that kicking a ball gives it a force that continues after ball and foot have lost contact.
The second law is quite different to the third. It relates the net force acting on an object to the object’s mass and acceleration: F = ma. This requires focusing on one object in isolation. “Equal and opposite” forces acting on the same object are balanced, and cancel each other out. In contrast, the forces in a third-law pair are always “equal and opposite” but do not cancel each other out because they act on different objects. Armed with an understanding of the third law, the second law becomes easier to apply and this leads to better learning outcomes.
In the Stile Science unit, Newton’s laws of motion, we present the laws in the order 1-3-2. We believe this provides better scaffolding for students as they struggle to overcome their misconceptions. Of course, other orderings can also be defended. The beauty of Stile is that teachers are free to modify any unit as they see fit, including re-ordering the sections. In any case, we should not feel constrained to teach Newton’s laws in the order in which he originally laid them down.
Science Lesson Writer & Editor at Stile
Brown, D. E. (1989). Students' concept of force: The importance of understanding Newton's third law. Physics Education 24, 353–358.
Low, D. & Wilson, K. (2017). Weight, the Normal force and Newton’s Third Law: dislodging a deeply embedded misconception. Teaching Science 3, 17-26. (link)
Low, D. J. & Wilson, K. F. (2017). The role of competing knowledge structures in undermining learning: Newton’s second and third laws. American Journal of Physics 85, 54–65.
Lutz, J., Sylvester, K., Oliver, K. & Herrington, D. (2017). 3, 2, 1…Discovering Newton’s Laws. The Physics Teacher 55, 149. (link)
Stocklmayer, S. M., Rayner, J. P., & Gore, M. M. (2012). Changing the order of Newton’s Laws: Why and how the Third Law should be first. The Physics Teacher, 50, 406-408. (link)
Terry, C. & Jones, G. (2007). Alternative frameworks: Newton’s third law and conceptual change. European Journal of Science Education, 8, 291-298. (link)