Sum of Forces = 0N Lab-Based Practice Sheet

I haven’t posted in a few days, so this is going to be a long one, covering three whole days of exciting BFPM work!

After finishing our main balanced forces practice sheet, I introduced the “sum of forces equation” notation. My hope is that students will work the kinks out of using it during the BFPM unit, and then the UFPM unit will be a cinch.

At the end of Tuesday’s class, we looked at a really simple example from the PhET Force Basics simulation (, involving three people in a tug of war. We’re given Fmedium on cart and F large on cart, but we don’t know Fsmall on cart. Finding the value is easy peasy for students, but it’s an opportunity to practice this new approach. The takeaways here are a) this equation is handy for finding values we don’t know, and b) we NEED to set a positive direction before the positive and negative signs have any meaning.

Then, we spend a day working on a practice sheet based around five lab stations, two of which are shown here. There’s also a falling coffee filters problem, a problem with an object supported from above and below, and a problem involving a collision between a bowling ball and bowling pin. For each, student have real stuff to play with and observe. They can use force meters to, for example, figure out that the water has to be pushing upwards on a block when it’s submerged, or make careful observations to see that the interaction that speeds up the bowling pin slows down the ball.

The two scales pushing upward plus the Earth pulling downward sum to 0N, as long as you’re not bouncing around.

The force exerted by the water on the cork balances the force exerted by the Earth exactly, but isn’t great enough to balance the force exerted by the Earth on the brass object.

Then, on the following day, we whiteboard the examples (with mistakes!), to get a solid idea of how to use this stuff. It goes a little quickly, and I wonder if it’d be better to spend even more time on the whiteboarding here, but it’s unquestionably awesome.

Students have the stuff with them when they’re presenting, so they can use it to illustrate their points. Simply having things to use as visual aids can contribute significantly to engagement while whiteboarding, especially if some of the audience didn’t get to play with that stuff.

##bfpm ##physicsfirst ##whiteboarding ##representations


Measuring vs. Inferring FEarth on Object

I tend to make a big deal of how Fscale on object is the only force we can measure with a spring scale. Any other values have to be inferred. I always wonder how much this ends up sinking in… the next time a student stands on a bathroom scale, are they thinking about inferring the force exerted by the Earth?

Today we did the classic lab where we found that the slope of the FEarth on object vs mass graph was 10 N/kg. one student was puzzled when she got a handout that had this relationship printed on it already. Why is it necessarily the same? Isn’t this the slope that we got for our experiment?

On the one hand, this reveals an incomplete understanding of the nature of the experiment we were carrying out, but it makes sense, since all the other experiments we’ve done have contained other parameters we could change.

I think I kind of glossed over how wild this result is. 10?! Without any understanding of where this number comes from, this must seem like a handy coincidence…

##bfpm ##equipment ##physicsfirst

Dueling Whiteboards Electrostatics

I made the decision to try dueling whiteboards with AP today, because we were low on time, and there were only two problems to work through.

The smaller groups worked well, and it felt way more like a conversation with the group who made the board, rather than a presentation. There are good things and bad things about this, of course, but it felt right for today.

This group got into a discussion of whether the point where the sum of forces on a positively charged particle is zero has to be the same point where the sum of forces on a negatively charged particle is zero. I’d thrown that part in as an afterthought, but it ended up resulting in some important symmetry argument.

Amazing how my whole “multiple representations” kick hasn’t exactly sunk in… This group barely drew a picture! If it wasn’t the AP, and we didn’t have months more material to “cover”, I’d be more of a stickler about this.

##ap ##whiteboarding

Horizontal Force Diagrams

After spending a bit of time with the newish PhET Force Basics simulation (check it out at ASAP if you haven’t seen it), I started thinking about the value of focusing on horizontal forces to isolate some of the big difficulties that come up around force and drawing force diagrams.

For example, the classic “Fball on ball” for an object moving with a constant velocity. This group originally asked if they were allowed to have “unnamed” forces, and I referred them back to our definition of force – an interaction between two objects.

Here’s another one. Again, these students didn’t like the idea that the wall was pushing on the ball, but they knew that there was an interaction there. So, they decided it must be Fball on wall. We didn’t get to discuss this one, but it’ll be a great first discussion when we get back to school after a four-day-long Presidents Day weekend.

Limiting the discussion to horizontal forces has been good, but it’s not perfect. More than ever before, I have some students this who really aren’t convinced that the floor pushes up on a ball when it’s just sitting there. I dealt with this by just encouraging students to look at the same problems again with vertical forces, and this was effective. It just takes a while!

##bfpm ##physicsfirst ##whiteboarding

Defining a System

Since we started the year studying energy situations with all objects in the system, we haven’t yet really identified what a system is in the first place. In fact, today was the first day we used the word.

Since we didn’t have an idea of force to work with, we couldn’t really discuss work (or at least, what’s the point if doing so if you’re not analyzing force?), and if we’re not discussing work then what’s the point of defining any objects outside the system?

This led to an interesting dynamic, as students immediately wondered why it was useful to leave objects out of the “system” in the first place. One student pointed out that we’d need to focus on a particular object in order to draw a motion map, so defining a system can be identifying that object, but some weren’t convinced.

I felt good about this, actually. I’ve got them wondering what a “system” is useful for, rather than just simply taking my word for it that’s it something that they’ve got to do.

##representations ##bfpm ##physicsfirst

Carts and Spaghetti

I discovered to my horror a few weeks ago that the bowling balls I’d picked up last year have gone missing from the school. (An odd thing to go missing, to be sure…) I had to come up with something similar for the bowling Ball and ruler activity, otherwise known as “Broomball.”

I’d discovered that a piece of spaghetti works nicely as a “force-minimizer”, as long as students hold one end of the spaghetti and push with the other. As with the rulers and the brooms, it’s important to see the spaghetti bend when there’s an interaction between the spaghetti and the cart.

I’ve modified the broomball sheet so that there’s a one-dimensional side and a two dimensional side. This meant that we only did the first side of the sheet, which was kind of better as an intro. One student pointed out – it’s just like with the PhET Skater, there’s a moment when the cart is totally still, in between when it’s moving right and moving left. Why yes, there is!!

##bfpm ##paradigmlab ##physicsfirst ##equipment ##yolo

CVPM in Flappy Bird

So, like many other physics teachers I’m sure, I finally got roped into downloading the insufferable, adorable “Flappy Bird” by reading @fnoschese’s post about calculating acceleration and impulse in the game.

We may yet get to do these measurements and calculations in my ninth grade physics class, but today we were finishing up CVPM, trying to figure out when the model is useful and when it definitely isn’t. So at the beginning of class today, I asked, “Can we use CVPM to describe Flappy Bird? Is he/she/it moving at a constant velocity?”

One student said yes, another said no, another said, amazingly, “Yes, but only sideways.” I got out two meter sticks and reminded the class that we’d talked about how position and velocity can be measured in two dimensions, and everyone seemed convinced that yes, CVPM was great for describing the horizontal motion on Flappy Bird. Not so much for the vertical motion, though, which is what makes the game so… popular?

(Notice the score, characteristic of the majority of my Flappy Bird games.)

##cvpm ##games ##pmm

Two Slits

In AP we looked at actual two-slit interference today. We’d seen diffraction gratings, but this was new. I used it as an opportunity to ask them to work with uncertainty, since the two slit light blobs can be pretty hazy.

Students predicted ranges were fairly close, but they were exactly the same.i told them that I’d take a picture of the slits under a microscope so we could check their work with a different type of measurement, and here’s what I came up with:

Those little stereo-vision dissecting scopes are amazing. I had to go into an elementary school room to find one, but they’re absolutely perfect for a lot of physics stuff. If you have a chance to grab one, don’t hesitate!!

Here’s a closer look at the students’ calculations. We all agreed that the 3 micrometer answer was suspiciously small, and the microscope confirmed our suspicions!

##ap ##equipment ##uncertainty