This is part of a series!
Part 1 (Work) Part 2 (energy bar charts) Part 3 (problem solving)
I have this lab I received from a colleague, it’s an iteration of a lab I’ve seen in other places. Basically an object goes down a ramp, gets caught by a paper catch/index card etc and students are looking for some iteration of work and energy.
In the version I have students are asked to find a relationship between height and distance. The cool thing about this is it ends up that height is directly proportional to distance and related by the coefficient of kinetic friction alone.
Student’s work looks like this:
Students are asked to complete the lab with a hot wheel car and then again with a small mass attached to the car. To students’ surprise the lines are not identical. This really bothers students until we discuss what we were actually looking for. See, the lines are still parallel, but the car with more mass is going to have a greater momentum at the bottom and will require a greater impulse to stop. It’s a fantastic conversation piece.
I really enjoy this lab because it requires students to consider a new problem and then apply that knowledge to a lab setting. Research has shown that students don’t really learn content in the lab, they learn lab skills. I was always a little frustrated with the disconnect between all of the work students put into the theory and then the lab results themselves. So this time I changed things up.
Instead of giving students the lab hand out and letting them work in groups, when students walked into the room they were put into visibly random groups. Visibly random grouping just means you create the random groups in front of students so they see it was truly random. I’ve been immersed in the book Building Thinking Classrooms and the research on this is really cool.
Once students are in their groups and at a white board that is vertically mounted, I’m in the middle of the room at a lab table with the lab set-up. I verbally explain the set up and that I want them to derive a mathematical model for the relationship between height and distance.
Vertical whiteboarding is really cool and has several advantages. First, students are standing which puts them into a more active position, this gets more of them working. Second, it’s really easy to just look around and snag ideas from other classmates. Third, since they’re already standing it’s really easy to move around the room and discuss with other groups. The first time I did this what astounded me was the sheer number of students talking. Instead of it being maybe 4 or 5 leaders it was nearly everyone in the room! There was so much collaboration and ownership of learning it was magical.
So I did this with the first part of the lab. Next, I asked them to sketch what the graph will look like with the two lines. Almost all of the students sketched the two lines on top of each other. I want them to have the experience of their data not aligning with their previous ideas and having to reconsider, so we left it at that. Then students were off.
I’m going to finish this lab this week, so I’ll have to come back to update this post, but I love this activity and vertical whiteboarding gets a 10/10 every time.
Physics.Teacher.Momma. 
Curious why the mass was cancelled in the energy analysis since the MgH is for the car mass, M, but the mu*mg*d mass, m, is for the ‘roadblock’ mass that the car crashes into and which slows the car, no?
These two are different masses I believe.
In that case I see
d = MgH/(mu*mg) which should yield a straight line through the origin regardless of car mass, but with a different slope (steeper the bigger M of car) when car mass is changed and graphing d vs H for various Hs.
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“Curious why the mass was cancelled in the energy analysis since the MgH is for the car mass, M, but the mu*mg*d mass, m, is for the ‘roadblock’ mass that the car crashes into and which slows the car, no?” Actually, it’s not. Friction is doing work on the car slowing it to a stop, so you would use the mass of the car in finding the normal force, not the roadblock.
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Jeff is correct that technically we need to consider the inelastic collision that occurs at the bottom of the track. The paper, of course, is so that we don’t have to worry about rotation in the activity. Yet since the mass of the paper is so small compared to the mass of the car we operate under the assumption that it is negligible.
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We run this lab under the assumption that the mass of the half sheet of paper is so small compared to the car that it is negligible. Should have included that in the description!
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My apologies Jeff- I didn’t really read your answer well. You’ve made an excellent point.
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I guess my question remains, how is this frictional force that stops the moving. At being generated? IF it’s actually the normal force balancing the car’s weight that is being used in mu*Fn, then the additional mass added to the cart should still cancel and you shouldn’t have a y-intercept.
If, however, the friction is due to the index card/piece of paper, then I’m not still not sure how the mu*Fn places a roll and what the Fn is.
The details of that frictional force part of the procedure are certainly critical.
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