Tag Archives: linear motion

Building The Net Force Particle Model (Part 1)

From “The How” to “The Why”:

One of the three projects that the students will complete this year is a custom designed and fabricated rocket. One of the requirements of this project is for the rockets to carry a small solid state altimeter that collects vertical position data. This year I decided to give the students some data collected by last year’s students. Here is what the data looks like from one typical altimeter reading:


As an introduction to this next model, I presented them with the data and asked them to use both the Constant Velocity Particle Model and the Constant Acceleration Particle Model to describe the motion of the rocket based on the data. Students responded to several questions that I created and they posted their answers through the Learning Management System we use.

A Simple Definition, A Simple Representation

The student investigation teams were then asked to draw velocity vs time graphs on their whiteboards. I was impressed to see that most teams were able to interpret the position data and create a velocity graph that agreed with the data. There was some debate about the graphs, but the students worked through these differences and came to consensus around what the graph would most likely look like. At this point I was thinking about using LoggerPro’s ability to graph the derivative of a data set, but decided that I would leave that for a later date, though next year I might do it earlier.

I then introduced a very basic definition of a force:

“A Force is A Push or A Pull”

And then I proposed that we could represent the force with an arrow, just as we had done with velocity and acceleration. I then asked them to divide the rocket data into four sections based on the answers to the questions we had discussed. The students then drew a representation of the rocket in each stage and the forces acting on the rocket. The stages the students identified were 4) on the ground, 3) descending by parachute, 2) going up without fuel, and 1) going up with fuel. I asked them to draw the diagrams by starting at the end. Here is a typical example of the force diagrams the students drew:

photo 2

The labeling is a standard that is outlined in the Modeling methodology – it reads (type, feeler, dealer).

Constant Velocity Motion and Net Force

We started the class discussion by looking at the forces acting on the rocket when the rocket was on the ground. Students agreed unanimously that there were two forces acting on the rocket – one down, one up – the gravitational force and then the force from the ground. Great. Then on to the descent phase. Certainly less unanimity here. The students again agreed on the number of forces – two – one up from air resistance, one down from gravity. The students quickly got into several back-and-forth arguments about the length of the force vectors. The class was split. Were the forces equal? Or, was gravity “winning”? The big stumbling block was around the question, “if gravity was equal to the air resistance force, then why was the rocket still falling”? A classic example of Aristotelian thinking. I encouraged them to ask the question – “if gravity was winning, why wasn’t the rocket speeding up?” One student proposed that maybe the force of gravity was just ever so slightly larger. Some students pounced in this. They argued that the forces weren’t equal at first, but as the rocket (with parachute) descended, the air resistance force strengthened and eventually became as strong as the gravitational force. the reason the rocket didn’t slow down was because it was already moving when the forces became equal. Awesome. Then a student gave an excellent description of a thought experiment where a box was traveling through space in one direction and convinced the students that the box would not slow down if you pushed equally on both sides of the box. Students reached consensus – the rocket moved at a constant velocity because the forces were equal.

The “Residue” Misconception

We then progressed to the next stage. Things got really interesting. Without exception, ALL the student groups identified an arrow pointing upward, even though they all agreed that the fuel had run out. The question that I think cuts through this the quickest is to ask “who is pushing on the rocket upward?” Most students get that funny look on their faces as their brains begin to realize that they just ran into a logical conundrum. Some students start to respond – “the rocket pushes the rocket.” OK, how? What kind of force is it? A contact force? How does it push or pull itself? The students at this point began to question each other and the room erupted in arguments. Being a bit of a control freak, I’ve had to learn to allow space and time for these chaotic moments, but also realize the importance of catching the class before it descends into something less productive.

At this point, one group erased the upward force. I asked them why they had done this. They responded that they didn’t think a force was needed for the rocket to continue upward, and that gravity and air resistance were slowing the rocket down. This seemed impossible to some of the students. They asked – “but something is left over after the fuel runs out, isn’t there?” The class began to divide up into those that now believed the rocket no longer had any upward force acting on it and those that believed there was some kind of “left-over” force, what I call a “residue”. So, once again, I asked them to identify the dealer of the residue force. The answer is generally – “the fuel”. Ah, but hasn’t the fuel run out? Yes, but the rocket has gained something from the fuel and now that is what is pushing it upward.

This is not such a wild idea, and in fact is not that far from the idea of Kinetic Energy. The students that were in the “no upward force” camp started to explain to the other students in the class that the fuel had “given” the rocket its upward velocity, but now that the fuel was gone, the rocket was now slowing down. We discussed the idea that anything that was slowing down must be experiencing a force pushing in the opposite direction of its velocity. We returned to the thought experiment with the box floating through space. The students debated about whether this box would slow down if the force that had gotten the box moving in the first place disappeared. The students agreed that if there were no forces acting on it to slow it, then it was reasonable to say that it would never slow down. Students then agreed the rocket was no different. It didn’t need a force to continue moving at a constant velocity, but that if it was instead accelerating (in this case in the negative direction) then it would need a force, which was provided by gravity and the air resistance force.  The students began to coalesce around the idea that if a force was a push or a pull, then the rocket that had run out of fuel was not getting pushed any longer, and that although it was moving upward it was indeed accelerating downward.

Making Some Observations

During the next class, I had the students set up a motion detector on one side of a Vernier dynamics track and use a force meter to pull on a low friction cart. They were to also record the velocity of the cart while the students pulled twice in quick succession on a string connected to the cart and force meter.


The students then shared their graphs with the rest of the class:


This experiment is meant to re-enforce some of the arguments made during the previous class. The students quickly see that the velocity is measured to change when the force is applied and that the velocity is “constantish” when no force is applied. The students were ready to tackle how the force and acceleration were quantitatively related, but that’s for another post…

Building The Net Torque Model – Part 1

A New Model (or Five New Models!)

So, since I decided to incorporate AP Physics into the Academy as the singular curricular pathway for all students, I needed to see what models might need to be added (and mostly removed) from the past curriculum. The big change was to remove the study of thermodynamics and much of magnetism, and add rotation. I hadn’t officially taught rotation since the Academy was first started nine years ago and since I had started using the modeling approach, so I looked at the curriculum AMTA had stored away in its repository. After reading through the material, I honestly have to say I don’t think the American Modeling Teachers Association’s curricular material on the topic of rotational motion is as good as some of the other material they offer. First of all, I have struggled with the idea of teaching this as a unit on Rotational Motion. It seems to go against the modeling approach of defining a distinct model that is either descriptive (kinematic), causal (force), or “conservational” (energy and momentum). The material seemed too similar to the traditional approach. Also, there seems to be some major holes in the curriculum (where are the notes describing the paradigm labs or the deployment practicums?) It would seem that in order to capture rotational motion as an analytical model you would need quite a few models if you were to follow the pattern developed in linear motion. You would need to define a Constant Angular Velocity Particle Model, a Constant Angular Acceleration Particle Model, a Net Torque Model (Net Balanced Torque and Unbalanced Torque), and then either two more models, or at least an addendum to both the Momentum Transfer Model and the Energy Transfer Model. Additionally, each of the linear models refer to the main constituent as a particle. So technically, if I wanted to remain true to the model naming convention, I’d have to use a name like Constant Angular Velocity Rigid Extended Body Model (CAVREBM). That seemed absurd. After some time, I decided to settle on the Net Torque (Rigid Body) Model or NTM. I felt that this model could cover kinematic descriptions, causal relationships between a net torque and a rigid body and then point to how the previous momentum and energy models needed to be adjusted. Here is how we first started to build this model…

Defining Angular Displacement and Angular Velocity

At the outset of our task to build this model, the students did a simple activity where they graphed the angle of rotation of a wooden disk with the distance the disk rolled across a flat surface. When they graphed the angle through which the wheel turned to the distance the wheel rolled, they found that the radius of the wheel was the slope. This meant that the linear displacement of a rolling object could be related to the angular displacement and likewise, the rate of these displacements were related through the radius.

Turning Effect

Once we had established some understanding around angular displacement, we started this model by investigating the conditions under which a rigid body’s rotational motion changed and when it didn’t change. I want to thank Sam for his post on how to begin investigating a turning effect.The students essentially followed this line of questioning and observing to establish a set of rules for defining when a rigid body’s rotational motion state will change. This lead the students to first make the claim that a change in “turning” was due to an unbalanced force acting on the wheel (especially those forces acting at a right angle to the radius!). The “turning” didn’t seem to change much when the forces acting on the wheel were balanced.

Uh, The Forces Aren’t Balanced, But The Thing Isn’t Turning?

The next class, I set up a simply “see-saw” with a meter stick and two different masses placed at different locations so that the meter stick didn’t rotate. I then asked the students if the forces were balanced. They immediately replied – “no”. So what gives? Students immediately saw that the Force Particle Models were not sufficient in dealing with rigid body motion, and many immediately suggested that the distance from the center of rotation also affected the “turning effect”. This discovery lead to a discussion about particles.

Extended Rigid Bodies vs. Particles

All the models that we had built in the past assumed that an object could be simplified by accepting that all the mass of the object could be identified as being located at a single point, namely the center of mass, because each particle of mass located in an object was experiencing the same linear motion state. The problem with a rotating object is that most points within a rotating object are actually traveling with different linear velocities (and accelerations). We needed a new way to define the basic constituent of this model. I suggested that we stick to as simple a definition as possible – a rigid extended body. This could be identified by a straight line (or lines) passing through a rotation point. The length represented an “un-bendable, un-squishable” collection of particles extending from the center of rotation outward to any edge of the extended body.

The Moment For The Moment Arm

This set us up for the next investigation. I asked the students to create an investigation where they had to prove that there was a connection between the force and the location that force was being applied and the turning effect. Students at first struggled to create an investigation that demonstrated a functional (input=output) relationship. Many students found evidence that supported their hypothesis, but I had to explain that these were not conclusive because it was limited to a single data point. This lead to a rich whiteboard session where students worked through the process of designing an experiment that went beyond describing a single situation. I’d like to return to this at a later point regarding scientific reasoning, but that is going to have to wait. Students eventually graphed the output force required when the input distance from the center of rotation was changed – in order for the system’s rotational motion state to be unchanged. This also quickly lead to the question of direction. It seemed that the force direction in relation to the radius of rotation seemed to affect the rotational state of the meter stick. At this point I introduced the concept of Torque and we discussed the vector cross product. I think for future classes, I’d like to introduce the cross product a bit differently. Using the line of action (as is done sometimes by engineers) seems to be a more useful method – at least visually – in explaining why the angle of the force is also part of what defines the torque value.

Deploying The Model (Part 1):

To test the model (thus far), the students were given a meter stick with a small weight attached to one end. The students then had to predict the point at which the meter stick could be moved off the edge of a table before rotating, and thus fall off. This allows the students to see the importance of identifying the gravitational force on the stick as affecting the turning effect around a point of rotation – which in this case was clearly not the center of mass of the stick. After some challenges, the students were mostly able to predict the point at which their meter sticks could be pushed before falling off the table. Next time, I’d like to reinforce this by also doing an experiment where two force meters support a meter stick with weights placed at different locations along the meter stick and have the students predict the forces read by both force meters. It was now time to look at situations where the torques were unbalanced. On to the second part of the model.