*Physics 11* – Students conducted a lab on Hooke’s Law today. They were prompted to model the relationship between an external force and how it affects a spring it is acting on. Data collection went pretty well and I strongly suggested that students use Excel to produce their graphs. Several groups opted to hand graph their data, which was very surprising to me. However, after pointing out a few things that they could do to improve their graphs, most of these groups eventually realized that re-doing it in Excel would be a lot of faster and probably more accurate.

# Tag: forces

## Day 88 – Forces Follow-Up

*Science 8* – Following from last day’s lesson, today students formalized their ideas on forces by reading through the textbook and answering some questions. The focus on this work was the differences between constant and non-contact forces, and what kind of motion results from unbalanced forces.

For the last 20 minutes of class, students were asked to put together a concept map for forces. I gave them keywords such as: balanced, unbalanced, constant, contact, at-a-distance, gravity, electrostatic, etc…

Most of the class nailed the concept map – they had sufficiently complex connections that made sense (indicating they knew how the concepts were related) and the links and descriptors that were accurate and applicable.

## Day 86 – Forces

*Science 8* – Today’s lesson has students pushing a hover disk back and forth along a desk. While doing this, students are asked to make observations and try to answer the question, “what forces are acting on the soccer disk, and what is the result of force?”

There are a few key things at play in this activity. First, I ask students to identify forces acting on the disk. Kids will have a variety of ideas from 1 to 4 different forces. The two most common forces identified are the push and gravity. We then break it down a bit by looking at a stationary disk on the desk. From this, students are convinced that there must be a force pushing the disk up (otherwise gravity would pull it down to the ground). Eventually we get the point that maybe there are 3 forces acting on the disk.

Next, I put some restrictions on when we are looking at the forces – I deliberately set it up that we are looking at the forces while the disk is moving between two markers, and no one is touching the disk at this point. Students will say there is a push on the disk. The next question then is, if there is a push, who/what is doing the pushing? Obviously the kid, they’ll say. But wait, how can this be if the kid isn’t touching the disk? This causes some serious reflection from the kids. Eventually we get to the idea that there are only two forces acting on the disk (gravity and normal force), that these forces are balanced, and that it results in constant motion. Whew.

This can be one of my favorite lessons because it forces students to really think about where forces come from, and what it means to have balanced forces. It is also a good challenge for getting the students to make useful observations. However, there is a big downside to the lesson. With a class of 30 students, it can be very hard for every student to have a voice. In particular, students that aren’t as quick as others to catch on will not truly participate.

## Day 77 – Goal-less Problem

*Physics 11* – Students had their first try at a goal-less problem today. In general, the students had done ok on their practice problems for unbalanced forces and quizzed well on it. To do more practice and something a bit more difficult, I introduced the goal-less problem. The idea is that students are presented with a situation and it’s their job to analyze the physics behind it. There is no one answer or thing to find, they just have to apply what they’ve learned. It’s also a good way to reintroduce kinematics to their problems.

## Day 73 – Unbalanced Forces

Physics 11 – Students were applying their understanding of force diagrams, Fnet and unbalanced forces to problems today. I usually don’t get too prescriptive on problem solving techniques, but I spent three days emphasizing #1 and #2 in the photo. As all teachers know, kids really resist doing things they think are not necessary. Unfortunately for them, skipping steps 1 and 2 will almost certainly result in mistakes even for the smartest kids with the most intuitive understanding of physics.

# The Big Tip for Physics Teachers

Ok, so we all know that kids try to skip steps. My students would often mix up writing out an equation for the unbalanced forces (Fnet equation, Fnet = ….) with Newton’s 2nd Law (Fnet = ma). Here is the new thing I found out this year: after kids draw and label their FBD, they become paralyzed on the next step because they realize they don’t know what all the forces are.

I used to think that they don’t write out Fnet = .. + … + … because they were trying to skip a step. What’s really happening is that when they start to write out Fnet = …. what they’re expecting is that they should already know everything on the right hand side of the equation. *They often don’t, and this becomes a discrepant event for them.*

There is a strong link between this and a phenomenon in math. In math, students often have a difficult time understanding the “=” in an equation. Many people don’t see equations as being a statement of equivalence. Instead, they see an equation as a question on the LHS (what is?) and the answer on the RHS. This is what happens with Fnet. The intuition is to have an answer for everything on the RHS. Note: just because students can give a definition of what the equals sign means, doesn’t mean they treat it like that when working with math or physics.

## DAY 71 – Elevator Lab

*Physics 11* – What does a coffee maker have to do with an elevator lab? Absolutely nothing, but it’s still very important. I got a new coffee making device for Christmas, called the Clever. It’s not that much different from ye old Melitta system. You slap a Melitta filter in the cone, add some fresh ground coffee and pour in hot water. The Clever does not let water through the bottom until it is placed on a cup though, so the coffee goes through an immersion before filtering. The Clever is half pour over, half French press. I was interested in it because I use a hand grinder and it’s easier if I do a course grind, which is what you use for a French press and Clever. However, unlike the French press, the filter removes all small sediment and particles from the coffee so that you end up with a clean crisp cup of Joe.

# Oh, Physics.

The class watched a video of a person standing on a scale while riding in an elevator. The video was focused on the scale readout. Students had to work out the person’s weight and normal forces acting on them, and draw force diagrams corresponding to when the person was still, moving at constant velocity and accelerating. It was challenging for them but very thought provoking and useful.

## Day 64 – Last Day of Lab

*Physics 11* – It was a long road, but today the students finally finished their unbalanced forces lab analysis. This took a lot more time than I had expected, and I would say that the work ethic has been pretty good. Lots of kids were getting a bit mixed up with their data and calculations, so there were several times where I would grab their data tables and quickly calculate acceleration and graph in Excel. I had thought about showing kids how to use Excel, Desmos or Plotly, but opted to stick with pencil and paper. Learning to use this kind of software would take another day of time. Heck, I even find Plotly and Desmos confusing at times.

I’m really wondering if it is worth all of this time to do a lab on unbalanced forces….

## Day 62 – DMV for Fnet Lab

*Physics 11* – Having become familiar with DMV for direct measurement, today I had the students analyze unbalanced forces as a substitute for the modified Atwood machine lab. The modified Atwood machine poses a few obstacles for understanding. First, it’s kind of hard to set up unless you have several sets of cart tracks and pulleys (we don’t). You also have to instruct students to keep the system mass the same (hand waving). You also need some decent technology to for measuring time and position (ie motion sensors or motion encoders), or your results go a bit sideways. I think ticker tape timers work quite ok for this type of lab, but only if the students have played and experimented with the timers before.

This year I wanted another try at doing the unbalanced forces lab so I went the direct measurement video path. The lab instructions were posted as seen above, and I introduced the DMV to them. To my surprise, many students continued to have problems finding the acceleration of the cart, they started collecting all sorts of data about position and time. We worked through it and by the end of the class all the groups had data, some groups had calculated acceleration and Fnet for each case, and some students even started graphing.

## Day 58 – Mini Review

*Physics 11* – We’ve covered a bunch of little topics to date in forces: Fg, Fs, FBD. I felt that today was time for some more practice, and I could tell from student questions while they were working away that this was a good time to go over some problems. We also did a quick activity with masses and spring gauges to emphasize the difference between mass and force of gravity.

## Day 56 – Fnet

*Physics 11* – Today the students combined some skills and understandings they’ve been working on: drawing a force diagram, determining if forces are balanced, and then finding unknown forces by knowing that Fnet = 0.