As explained yesterday, today was about practicing with the charge model. We were still WB’ing the practice sheet, but that went hand-in-hand with drawing a series of pictures for conduction and induction, and each student having an opportunity to charge an electroscope with each method:
Well, I really like the Hot Wheels activity for energy conservation… after all, who does not love to play with the little cars? The issue for some today was that the kinetic energy at the bottom of the ramp, had a greater value than the gravitational energy at the op of ramp… not so Hot. So is the error with the determination of the change in height or the velocity?
I think I figured out the issue. I was using a single photo gate with ‘One Gate Timing’ to get the instantaneous velocity of the car at the final height. I’ve used a single photo gate successfully over and over and over and over …. it’s one of my favorite uses of the photogate. When I run the experiment, no issues, but when some of my generals did it, they must have measured the ‘flag’ incorrectly. It is a narrow flag to give a better value for the instantaneous velocity, but even a millimeter off makes a difference when the energy values are so small to start with. Tomorrow I will try a different photogate, one with no flag needed… a Bee-Spy photogate. I am pretty sure the cars will fit through. Stay tuned…
As mentioned yesterday, today was all about gathering data for the ‘ramp’ lab. This is the (usual) experiment that develops several concepts, that I’ll discuss tomorrow. I have set this up a number of different ways. This year I gave the students much more freedom and choice that I have in the past. They could choose from 3 different objects to send down the ramp. Their choices were a hot wheels car on its track, a constant acceleration dune buggy (this is the pull back kind, but they free wheel if you do not wind them up), or a large steel marble on a channel made from physics (duck) taping to 10 foot pieces of conduit together. These are the same pieces of conduit I use a guard rails for our buggy bash. I like the conduit ramps because it really nicely extend the ramp so the students can get more data, and allows for shallow angles to make the data easier to gate.
In a sense, we repeated the position vs. time experiment we did in the cv unit.
SIDEBAR: When we introduce the lab, it is not one of the ‘does the cv model still work in this situation” intro’s. I do not do this because I have found the majority of my students already know the cv model will not apply.
I like the similarities because we are still measuring positions and times to describe the motion. The students were also able to choose a method of timing; stopwatches or photogates (Pulse Timer – Two Photogates). It totally blows me away that after all we have talked about in terms of our inability to make accurate time measurements by hand, some groups still reach for the stopwatches first. Maybe it’s because they know them or think it is easier. The only condition I set is this: the odds groups (I use playing cards to group my students into lab grops) will put the origin or ‘0’ at the bottom of the ramp and start at some initial positive position, while the evens will put ‘0’ at the top and roll in the positive direction. I love this when it comes to whiteboarding tomorrow. It allows me to see who is still stuck on distance travelled instead of position and it awesome when they try to explain the meaning of the slope of the linear graph… stay tuned….
Lens and refraction test today… nothing too exciting. Tomorrow we start in on reflection and plane mirrors.