This semester, I spent a lot more time upfront helping students to develop sense of what force is, how we see evidence of it happening, and how to productively talk about it.
Here were some of our goals /process for day one (3 hours):
First, identifying interactions through “contact between objects that results in visible compression, stretch, or shear.”, then through the use of “bridging analogies” inferring the happening of contact forces even when the amount of compression, stretch, or shear make be nearly imperceptible. For this students had a variety of objects to see compression (springs), tension (rubber bands), and sheer (combs).
After talking about them, I had stations set up where each force could be visibly identified, and students had to make a claim at each station about one contact force that was happening, and to identify which object was causing the force, where it was occurring, and the evidence for the type of interaction. The stations included:
- An object suspended by two rubber bands (both stretched)
- An object suspended by two springs (one compressed, one stretched)
- An object sitting on a brush (with both sheer and compression) being pulled by a rubber band (stretch).
- A buggy spinning its tires on a brush pressing (sheer and compression) into a ruler (compression).
- A fan cart with fan turned on (with tinsel tied on the fan cover), pressing into a hoop spring (compression), resting on a bridge made of two rulers (which show compression).
- An object on a ramp supported on two sides by foam (both showing compression).
Second, our goal was to practice the language of force as a happening. We talked about parties, parties occur at a particular location, they start at a certain time, they persist for an amount of time, they end at a certain time. During the time of the party, the intensity of the party can range from mild to wild. When the parties is over, the participants can’t take the party home with them. Partying may have caused the participants to have changed their state (e.g., from sober to inebriated), and both the duration and intensity of partying can effect the amount of inebriation.
And so, it is the same with forces. A “forcing” between two objects, occurs at a particular location (the boundary between two objects where contact is made), they “forcing” starts at a particular moment, it persists for a certain amount of time, during which the force can range from gentle to intense, and then the “forcing” between objects come to a rest. The “forcing” event may have caused the object to change, but the object cannot take the “forcing” event with it.
Students were asked to be playful in investigating the claim: “During the time that an object is experiencing “forcing”, the object experiences a change to it’s motion.” They were asked to look at two changes specifically: “rest –> moving “, “moving –> rest (or possibly even turning around)”. They were asked to use a their finger, a coil spring, a hoop spring, a rubber band, and a comb, guiding questions asked them to talk about force in a productive way, including
- Identifying the object that was doing the forcing, and the object that experiences the forcing, and to identify evidence point to a specific type of interaction (compression, shear, or stretch).
- Identify the specific location where the forcing was event was occurring (e.g, the forcing was occurring between the tip of the comb’s blades and the rubber side of the cart).
- To identify the moment when forcing began, when it ended, and to consider whether the intensity of the forcing event more “gentle” or more “intense”, and whether the duration seemed “brief or extended” in time.
After introducing magnetic and gravitational quickly as an example non-contact force (to be talked about in more detail later), students were given more practice identifying interactions where evidence is less perceptible (e.g., cable pulling an elevator, bicycle descending a hill, a car parked on a hill.
We ended the day discussing this clicker question:
One of our low-friction cart is at rest on a long track. The cart starts at rest, and then you at time t1, you press your finger against the car, maintaining a constant amount of pressure with the finger the remaining time. What will a velocity vs. time graph look like?
[I motivated this question by saying, we see evidence that “forcing” can caused an object to change it’s motion, but how do we think that change occurs”
Last year, we some really great conversations about this question. And this year we did, too, but this year the conversations were entirely different, because of the time we spent talking about evidence/mechanisms of forces. Students were much more attuned to the feature of the question that said, “keeping constant pressure”
At first vote, we were all over the place. Many thought that the two graphs that leveled out were possible, depending on how quickly it got up to speed. But a growing contention of students were making a compelling case that when you first press against the cart (it tries to move away from you, which would naturally release some of the pressure, so you have to keep up with the cart to maintain constant pressure). To them that suggested that what naturally happens, most of the time, is when you press on something, you don’t actually exert a constant amount of press, and the in fact, a constant pressing should result in non-stop speeding up, with the cart speeding up to get away from you, and you speeding up to keep the pressure. Upon revoting, more and more students were thinking it had to be one of the two graphs showing speeding up the whole time.
I had planned it, but I got out the hover pucks, and asked students to go out in the hallway and investigate the idea that when you press against something, you have to keeping speeding up with the object to maintain a constant pressure. Everyone came back confirming that this definitely seemed to be the case… and wanting to know which of the two graphs it would actually be.
I might have waited until the next day, but we ended up getting out a fan cart at the end up and looking at the motion detector, showing that a steady fan push resulted in a steadily increasing velocity. The following day, we would return to investigate this more closely, which I tell about later.
Thanks for writing such a helpful blog. I was thinking about stealing some of your good ideas for my class and am in the process of thinking through it. I am a little bit confused about the idea of the stretch category of interaction. Let’s say you have a mass hanging from a spring. Although the spring is stretched, at the location of contact, isn’t the spring compressing (a microscopic amount) the hook of the mass? Maybe overthinking this, but at the point of contact, is the mass also compressing the tiny part of the spring that it is actually touching? Do you think that is worth going into with students? Thanks.
Yeah, and it almost always comes up from students. This year students decided that holding onto a rope was shear (friction), not tension. The decision always comes down to details of the boundary of system you choose. I have another post that talks about this issue here: https://teachbrianteach.wordpress.com/2013/10/26/tension-or-normal-force/
I’m guessing that the red strings are intended to indicated the system boundary you want students to focus on?
Also, how do you secure the brush with the mass on it? Is duct tape sufficient? Do you have students hold onto it?
Yes, String is for system boundary. I use a table clamp with the Brush.
Can I just say, thank you a million times for posting these types of things for free. Ive been teaching for ten years and I love to do modeling/student centered approaches. This is my first year teaching physics and you, Frank Noschese, Kelly Oshea, Marta Stoekel, and Joe Cossette are making a very daunting process of planning seem much less scary. I really appreciate your willingness to share ideas. Hoping to pay it forward once I get my feet wet. Thanks again!