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 74: Universal Gravitation


Physics 11 – Today the students worked through the formula for Universal Gravitation.  The students felt pretty good about how gravity works, having covered it conceptually already.  However, there were still a few surprises (mostly for me).

It became apparent to me that many students consider the re-ordering of variables in a formula as something that makes a new formula.  For example, after rearranging the formula to solve for r, separation, I was asked several times if the students would be given this formula on the formula sheet.

The second big surprise was being asked if the N*m^2/kg^2 units for G were variables, and what numbers should be plugged into them.

The above two misconceptions will have to be something that I specifically address again the future.

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.

Day 64: 2nd Law Quiz


Physics 11 – Today we had a quiz on Newton’s 2nd Law.  The idea was to quiz the basic Fnet type of situation, to see where all the students are at.  Doing a worksheet while chatting with friends can be a lot different from doing a quiz where there is no contact with others.

From our previous practice questions, the students were a bit mixed up with creating an Fnet equation.  For example, they still held the idea that they should memorize, or be given, the equation Fnet = Fn – Fg

I believe overcoming this misconception to be the biggest take-aways from the class.

Day 60: Applying Fnet Model to Problems


Physics 11 – Today the students were given a handout to summarize all the types of forces they have learned up to now.  I then prompted the students to solve some word problems, and went over the solutions after everyone had tried them.  By “everyone” and “tried”, I really mean that.  I’m well aware the studenting that happens in high school.  Many students sit there, fake it, or take bathroom breaks, so that they can simply copy down the correct answer once I go over it.  The solution?

  • Walking around, the classroom, help the ones that are truly stuck
  • Mostly leave alone the ones that are getting (but still check in)
  • Finally reveal an answer once everyone has seriously tried the question

I even heard students say they were going to wait until I go over it.  I’m not sure what they would learn from that.  Surely me going over it is no different from reading a textbook, or close to it.

Day 58: Unbalanced Forces Results


Physics 11 – Today the students wrote up their results from their lab for Newton’s 2nd Law. Many groups were getting close to developing a model for Newton’s 2nd Law: Fnet = ma.  Since the input for the lab was a force though, the students graphed a = Fnet/m

Once the slope was written as a fraction, the students were able to recognize that it was the inverse of the mass.

I have two classes of physics 11.  During the first class, I walked along a lot and spoke with students as they worked.  As a result, by the time we got to share the whiteboards, almost everyone had the same board. For example, I challenged groups to find better symbols to use, rather than x and y.  The second class I helped a lot less and the following discussion was better.

After our discussion, the students applied their new model to the situation of someone standing on a scale inside an elevator.  This allowed for some thoughtful thinking on adding forces together from well drawn force diagrams.

Day 56: Unbalanced Forces Lab


Physics 11 – Today the classes did the main modeling lab for unbalanced forces and Newton’s Second Law.  I’ve used the modified atwood lab in the past, but wasn’t satisfied with it due to friction and hand-waving with transferring mass from cart to hanging mass.  I also tried to setup an atwood apparatus as described by Josh Gates.  In this setup, you hang a mass from one side of the pulley, and on the other side you hang another mass via a force sensor.  If you time how long it takes for the force sensor to travel a certain distance, you can calculate its acceleration.  You get the Fnet directly from the force sensor. I also found this lab setup to be difficult, particularly because it really works best with a wireless force sensor.  In the end I opted to have the students get data directly from a force and motion simulation.

Prior to starting the lab, we qualitatively discussed unbalanced forces via Peer Instruction.  I did this instead of the suggested modeling lab with qualitatively measurement of pulling a person on a cart.  PI afforded some good discussion and I’m relatively confident that students understood the idea of unbalanced forces causing an acceleration.