This class is using Science Fusion Module I as in currently in progress. 2017-2018.
This class is using Science Fusion Module I as in currently in progress. 2017-2018.
Students were told to read Section 2.2: Wheels in How Things Work: The Physics of Everyday Life or Chapter 7: More About Forces in the Cartoon Guide to Physics. I requested the students watch the first 5 minutes of Crash Course Physics on Friction.
And some videos by Doc Schuster:
We pretty much jumped right into the lab activity today, which was to measure the coefficients of static friction, μs, and kinetic friction, μk, for a block on a surface (table cloth, cardboard, rough wood, whatever). There were a couple of ways students could make the measurements. The first involved placing a block on the surface and tying a string from the block to a weight hanging over a pulley. The amount of weight hanging off the string is the pulling force on the block. If the block is not moving (a = 0, therefore net F = 0) then the force of friction must be equal and opposite to the pulling force. Likewise if the block is not accelerating up or down then the normal force is equal and opposite to the force of gravity, the block’s weight. The weight of the block is measured in Newtons on a spring scale. If we had been doing the experiment on a ramp, then the normal force would NOT be equal to the force of gravity, we’d have to take into account the angle of the ramp – but today we just dragged the blocks horizontally. To measure the coefficent of static friction the students had to find the maximum pulling force they could exert on the block WITHOUT making it move. Once the block starts moving the force of friction is now kinetic friction, and is less. We’re trying to find the maximum force of static friction, Fstatic = μsFNormal, how much force do we have to overcome to make the block move. When they had that force, roughly 1N, they divided it by the normal force of the block to find the coefficient of static friction. The coefficient of frictions, static and kinetic, depend on the two surfaces. If the block is rough like sandpaper and is being dragged over a rough wood board, then the friction is going to be higher than if the block has been sanded smooth and is being dragged on a polished surface.
The coefficient of kinetic friction should be less than static friction, its easier to keep an object moving then to get it moving in the first place. Think about trying to move something heavy like a fridge, at first it won’t budget until you exert a very large force, but once you have it moving its easier and you don’t have to push so hard. To measure the force of kinetic friction, students dragged the block on the same surface at constant speed with a spring scale. If the block is at constant speed (at least roughly constant), then its acceleration and net force must be zero, so the force measured on the spring scale is equal (opposite direction) to the force of friction on the block.
Just as before they can now find the coefficient of kinetic friction, Fkinetic = μkFNormal by dividing the force on the spring scale by the normal force (weight they measured earlier) and indeed it was less than the coefficient of static friction.
The third measurement involved using the Go Direct Force Sensor from Vernier. Its basically a digital spring scale that sends the data directly to your iPad via bluetooth. Using this sensor students collected data while they slowly increased the force on the block until it started moving and then tried to keep it moving at a constant speed.
The graph of the data looks like this:
As the student increases the force on the block the Go Direct sensor records the increasing force, indicated above with the red line, but the block has NOT started moving yet. As the force reaches 0.72 N, the block starts moving and the force drops to 0.52N (green line). From this one data set students can get read off the forces required for both the static and kinetic coefficients of friction.
We didn’t travel to see the eclipse but did get to see 75% of the sun blocked by the moon and the morning fog burned off just in time to make it visible. We had eclipse glasses, cheap cardboard ones and some plastic ones, as well as pinhole box viewers, telescopes with solar filters and binoculars set up to project the image. My younger son even used a saltine cracker as a pinhole projector.
As the sun was coming back out we got a nice view of the sunspots through the telescope and projected by the binoculars
I also used my new Go Direct Temperature Probe from Vernier which connects to my iPad to take temperature readings in the shade for an hour before and an hour after the maximum eclipse. This was part of a citizen’s science project by the Globe Observer program/App.
We had a pretty heavy marine layer/clouds until about 9:30 which kept the temperatures down this morning. But you can see the temperature dipped a bit as the eclipse started and the clouds were burning off so I think there’s definitely a decrease in temperature caused the eclipse.
Great morning full of science. I’m a bit envious of everyone who was in the path of totality but we just had too obstacles for traveling and it was nice being able to just step outside and view it in the backyard. We did notice that in 2024 my parents house will be in the path of totality, as will Kenyon College, where both my husband and I got our bachelors in Physics AND UT Austin where we attended grad school will also be in the path, so we’l try to make it to one of those!
We had pretty much finished all the ‘interesting’ labs in the ACS Middle School Chemistry curriculum so I dug out a forensic science kit that a friend had purchased for us a year or so ago. Its one of many kits in the The Mystery of Lyle and Louise, a hands-on forensic science curriculum. Its made for high school students but the kit I have is for bite impressions, Lyle and Louise Bad Impression Bite Marks Analysis Kit, and the kids just had to measure a few distances to compare bites so it was ok for middle school. We actually did this in two classes. During the first class I showed them a slide show I found on the internet about forensic odontology (using teeth/bite marks in forensics) and then everyone made a bite impression in a piece of wax and learned how and where to measure on the bite impressions.
For the second class I read them the story of the mystery of Lyle and Louise which involves a car wreck, a murder scene, a drug bust and various other bits. There is a slideshow you can find on the internet premade for this. I made sure to tell the kids this was a made up story and got the ok from parents before showing the slideshow – all the crime scenes are drawn images so its not graphic or anything. Each kit you can buy tests different evidence for the same story, so to figure out what happened to the victims you have to do a lot of the kits if not all of them. The bite mark analysis that we did in class was just to see if one suspect was lying about the bite mark on his arm – did he get it from a guy in the bar or did the victim bite him? Between classes I took the bite impressions the kids made and separated them into different groups and added the impression that matched the photo evidence, both provided in the kit. Students had to make measurements from the bite mark on an arm (photo) and then measure the various bite impressions to determine who could have cause the mark on the arm. There is a spreadsheet you can download to do an analysis of the data – finding the set of measurements that match the photo the best. I entered the data for each group and it was interesting that two of them had 2 bite impressions that were pretty close to the photo so I told them to look at the photo closely for identifying marks like crooked teeth or missing teeth that would help them make a decision.
This was a pretty interesting lab and I wish the kits weren’t so expensive so we could buy more. This one classroom kit was $129. I did see that they now sell a small classroom edition that contains all the experiments for a little over $300 and its good for 6 kids. So it might work well for a homeschool co-op, everyone pitch in $60 to buy the kit and do the labs together. When you buy a kit you get access to their online resources.
Overall nice lab kit, just wish the price was lower.
For this lab I basically followed the Modern Chemistry lab: Household Acids & Bases but added a few things. The lab is in the textbook and can be found in the online resources. Its a simple enough lab with students testing various household items like dish soap, soda, lemon juice, vinegar, bleach, milk of magnesium, etc. to see if they are acids or bases. The lab called for us to make a pH indicator out of red cabbage so I did that the night before to save time. Its very easy to make, you just chop up a red cabbage and place it in a pan full of water. Bring it to a boil then turn off the heat and let it cool. Strain it, collecting the lovely purple water which is your pH indicator. I found this great photo below that shows the color of the indicator for different pH.
I also happen to have a pH meter, litmus paper (red and blue) and regular pH paper, so I had the students test 5 different chemicals with as many methods as they could. I also asked that they try to get a rainbow of colors with the cabbage indicator like in the photo above. Here are some of their results.
This lab worked really nicely and the color changes were pretty dramatic. Bleach turned the purple indicator a dark brown which quickly faded to yellow and eventually went clear (see photo with test tubes). In most cases the pH meter reinforced the pH values found with the cabbage indicator and the pH paper (which turns color like the cabbage indicator). Litmus paper just tells you whether you have an acid or base.
I had the students watch two Crash Course videos before class.
Here’s a good video on sulfuric acid:
We had some extra time at the end of the lab so I demonstrate the power of hydrochloric acid by using it to dissolve aluminum foil, which is the same demo I talked about in yesterday’s post for the intro class. The photo below shows the foil in muriatic acid (HCl) and the little bubbles of hydrogen gas starting to form.
In this video you can see the hydrogen gas escaping and the acid looks like its boiling, but the beaker is very cold to the touch, its just a vigorous chemical reaction producing a lot of gas bubbles. Near the end of the video you can see that the Al foil disappears completely.
Learn from Yesterday, live for today, hope for tomorrow. The important thing is not stop questioning ~Albert Einstein
because some of us need a few more lines to keep everything straight
Wonders of Physics
musings on life as a university professor
George Lakoff has retired as Distinguished Professor of Cognitive Science and Linguistics at the University of California at Berkeley. He is now Director of the Center for the Neural Mind & Society (cnms.berkeley.edu).