Every year I teach Newton’s laws in physics and most of the students are able to grasp the first and the second law. However, Newton’s third law is the one that is most contrary to their everyday intuition. This law as the students know it is “every action has an equal and opposite reaction”. The students have learned it since their first science classes.

But if I probe more deeply about it through scenarios of different collisions between cars with different masses and velocities, students do not apply Newton’s 3rd law any more. And they do so due to their everyday experience of seeing collision pictures where the smaller cars are completely totaled and the bigger car has minor damage.

So, this year, I decided to take a different approach to teaching this law in physics. The lesson started with having two students pull on each other with spring scales. I asked the students to pull so that the two spring scales read different forces. The students were unable to do so: each time one pulled with a certain force, the other spring scale read the same force.

Normally, students agree when I tell them that pulling with the spring scales attached and pulling hand to hand are the same except spring scales just reads the strength of the pull. This year, however, the students completely disagreed and said that using the spring scales make the forces the same.

So I decided to use Vernier Force Sensors. I had the students set up an air track and two carts with motion sensors attached to them. I asked the students to come up with different collisions and predict how the forces will compare to each other based on the collisions. The students changed the carts’ mass and velocity so the collisions were different. In the end, they saw on the force sensor software that the readings were identical from the two sensors.

One student summarized their learning from that lesson as “when two things collide with each other, they exert the same force on each other regardless of their mass and how fast they are coming at each other”.

To the right is a screenshot of the convincing data of two students pulling on the force probes from two different directions, each time the measured forces were identical regardless of who pulled harder. I believe that this year the students have a much better understanding of Newton’s third law instead of mere memorization of the phrase “every action has an equal and opposite reaction”.

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