Day 75 – Newton’s Third Law


Physics 11 – Today in physics we formalized Newton’s 3rd Law.  Students had already worked out equal and opposite forces when discussing gravity, but we didn’t put a name to it or generalize it to all forces. We also watched a video of two carts colliding, where each cart had a steel hoop attached to the front.  When they collide, the hoops on both carts compress the same amount. By recalling Hooke’s Law, the students reasoned that since the compression was the same, the forces had to be the same.

Day 68: Peer Instruction on 3rd Law


Physics 11 – Having discussed and debated about Newton’s 3rd Law last day, today was a chance to put the students on the hot seat with Peer Instruction.  Class #1: first vote was about 70% correct, moving to everyone correct but one person on the second vote.  Class #2: first vote had over 80% correct, no second vote required.


This question was more difficult because the answer is more subtle.  Class #1: first vote was about 60% correct, moving to 75% correct on the second vote.  Class #2: first vote was about 60% correct moving to 90% incorrect on the second vote.

I could see how students were convincing with their incorrect arguments.  The key was in the drawing of a force diagram, but very few students wanted to do this.  However, as time moves on I think I am slowing convincing students just how important a force diagram is.  Now that we are doing more word problems (5e: extend), I am reinforcing the idea that physics covers the force diagram and writing an equation for the net force, and then the rest of the solution is math.  By skipping over force diagrams, the students are not demonstrating understanding of physics.

Day 66: Newton’s 3rd Law

Physics 11 – We started off with a short look back at the conceptual work we did with gravity.  In particular, we reviewed how the gravitiational force between two objects is the same for both objects.  This was demonstrated using some scaffolded models and a tug-a-war with force gauges.  This work set a good foundation for Newton’s 3rd Law.

Having covered spring forces and gravity, the above video gave the students a convincing argument that interaction pairs have equal and opposite forces, regardless of the masses involved.  Once again, many thanks to Frank Noschese for his invaluable sharing of ideas, experience and resources.

Next we came back to how we do Force Diagrams (Free Body Diagrams):


The students were now given a second reason on why they’re asked to label forces with the details of “_____ on _____”.  The first reason given was that if they student couldn’t figure out the agent that was causing the force, there is a good chance that they are imagining a force that doesn’t exist.  Today, the second reason given is that this description clearly outlines the interaction pair.