Archive for the ‘Demonstrations’ Category
My father just sent me this TED Talk. He doesn’t read my blog and didn’t know about the other TED Talks I posted. This one is a little different, Ramesh Raskar from MIT has developed a camera that can slow motion down to the point of being able to see a pulse of light travel. You just have to see it to believe it.
And in case you aren’t seeing the embedded video:
This is not a new topic for me, it’s been a burr in my saddle for some time now. All of the introductory physics textbooks address significant figures in much the same way. The problem is – nobody in the “real world” uses sig figs. At the same time, introductory physics isn’t the time to introduce complex error analysis models.
I’m having this discussion with Andy Rundquist of Hamline University. I asked Andy how they handled this at the college level. He told me they don’t teach significant figures and pointed me to a very lengthy article discussing why significant figures are all wrong. The article suggests the use of Monte Carlo analysis its place. That may make sense on a lab, but not on classwork and homework problems. The uncertainty article did have a suggestion; use six significant figures for calculations and round the final answer to three sig figs. The article does a good job explaining the reasoning, and I’m fine with it. The three extra “guard digits” preserve the accuracy, and the rounding makes the answer more reasonable.
- I will project an archery target on the board.
- Students will move back about 20 feet and shoot a round of Nerf darts at the target. They will be far enough back that most of them will shoot a 6, or 7 and not a 9 or 10, at least at first. Each student will take a turn.
- We will plot the overall results. We should get something resembling a normal distribution curve, but I won’t tell them that.
- I will ask the kids to average the data and come up with a value of x.x +/- y.y and start a discussion on whether or not that represents the data.
- We will then put a ring or other object on an electronic scale and write the mass with the error in the same way.
- After some discussion, I will bring up slides of normal, rectangular, triangular, and maybe exponential distribution curves. I want them to discuss the fit of the models to the data.
- My goal is that they understand that error is probability.
- About a week later we will drop rulers and calculate individual reaction times. This would be a good time to bring back the distribution graphs and perhaps even input our data into a statistical analysis program to find the best fit.
I think this will work and go over well. I’d love some feedback. It’s a first pass, what did I miss?
I don’t remember seeing these before. I just got an email that the link to the Java Applet for acceleration had failed. It didn’t take me long to find the gentleman’s page and I realized that he has an extensive list of these physics applets. Here is the main page in English:
There are some really useful tools here for demonstrating mechanics to your students. Best of all, he has a download button, you can just put them all on your computer and not worry about his link changing. I haven’t been able to make the downloaded applets work, if someone can post some help, it would be appreciated. I tried just clicking on them, they open up Firefox, but they all seem to fail.
If you have access to computers in your classroom, you can design lab experiments where the students input conditions into these applets and read the results from the screen. Not as much fun as hands-on, but a lot better than lecture. If you do create an applet lab, please forward it to me, I’d love to attach it to this post.
I was discussing Newton’s Laws and trying to explain how the tension increases in an elevator cable when it starts moving up. The kids get it that the tension when it’s not moving is equal to the weight of the load, but once it starts moving, they get wacky. Some seem to think the only weight at that point is the force from F=ma and the elevator is now weightless.
I put 1 kg mass on the large spring scale and showed how it pins the reading if I pull up. They saw it, but it didn’t click.
On a whim a took I put the 1 kg mass and lifted it with some string. My standard classroom string is macrame string from A. C. Moore. It costs about $3 for 1000 ft ball of string. I think I go through a ball of it every year. Anyway, the 10 N weight is nearing the limit of what the string can hold. I accelerated the string upward just like I did with the scale and the string snapped. Watch your toes, it fell to the floor and cracked a tile… oops.
I came up with this in my early years of teaching (2002) and I forgot about it until tonight. At the time, I was borrowing a friend’s physics classroom to do a graduate assignment on reading. I prepared an article on how fireworks produce light and color. The concept I was trying to get across was how electrons in the shell jump to a higher energy level, then give off light as they drop down to a ground state.
I chose as a prop a Zippo lighter without fuel, but in the little basket on top I placed some magician’s flash cotton. This stuff is great, you can purchase it at a local magic shop. It is stored wet so that it doesn’t ignite, you take out only as much as you think you will need ahead of time and let it air dry.
I used one of the students in the class, he was the electron. The idea was that as he got heated up by the thermal energy, he got elevated into an excited state. He then had to stand on one of the desk chairs to show the elevated state. Now the problem was that he needed to get down, but he couldn’t without releasing some energy. To do that, he had to flick the Zippo and step down. As he flicked the Zippo, there was an impressive flash of light – the photon being release from the electron in a high state.
I know this worked, I heard a bunch of involuntary utterances of “I get it” and “ooh.” I think that was the day I knew I belonged in the classroom.
Fire – good.
I admit I’ve been holding out on you. Let’s just say I thought this one was a bit too low level. I beefed it up and it’s perfect for my Conceptual Physics classes now. I just did it this week and I like the results.
See that rocket shaped balloon on the right… good luck finding them. I seem to only be able to find the regular party balloons. Okay, I didn’t look that hard, but if they were at the local dollar store, I would definitely buy them.
So what’s the big deal about this lab, it seems almost grade school level? First off, again, almost none of my students have ever built a balloon rocket. This never ceases to amaze me. Second, it’s deceptively challenging. Lastly, the kids actually get the principles because of this activity.
To make this work in the classroom, I use a ringstand on one end with several book on the base for weight. I tie a string and put it across the room, leaving the other end open so they can put straws on the string. Each team gets their own ringstand/string setup.
The lab is broken into two missions. The first mission is easy; first, make it go across the room. Then put the balloon at 45 degrees to the string and see what happens. Then repeat for 90 degrees. They sometimes guess that the balloon will spiral, most are surprised but figure out why it spirals.
Mission two is really difficult, I tell them they are being challenged, but they are not graded on success, they are graded on effort and documentation. The mission is to make the balloon go down and automatically come back. They are told they can use two balloons. Once they get into this mission, the kids tend to put two balloons facing opposite each other and let go at the same time. They seem to think that the balloons will know to take turns. They learn first hand that the opposing forces cancel each other out. That’s really the gem in this lesson. I don’t care about them making it come back.
What most do next is blow up one balloon bigger than the other thinking it will move the way they want and then come back because it lasts longer. Again, they usually figure out that the opposing forces cancel. They next try to delay the release of one of the balloons. I get a few creative ideas here using bent straws and twisted balloons, but so far no amazing designs.
I like to sit back and watch this one. You can see the lights go on when they figure out the opposing forces cancel. Below is the lab handout. If you get some good solutions to the return mission, I’d love for you to post the solution here.
I kind of made this one up, kind of adapted it from the electronic timer manual. The idea is that we use a ramp to accelerate a steel marble, have it pass through timer gates, measure the distance between the gates and calculate the velocity. Do that a couple of times for accuracy.
Now measure the height of the table using a meter stick. Use a fishing weight on a string to find the point directly under the edge of the table. We now have the horizontal velocity, the height of the table, we can calculate how long it will take to fall. Next we do the math and place a penny where the steel marble should land.
In the beginning, it would works sometimes, but not always. I determined that our heavy epoxy table tops caused the steel ball to bounce, losing some of its horizontal velocity. The bounce was easily dampened by placing two or three sheets of paper under the end of the ramp. We also had some issues with hitting the side of the photogate. Lining up the gates with the ramp was a minor issue, but an important one. After the speed was determine, we moved the photogate away used the already calculated speed.
Here is the Lab as I wrote it up, it needs some updates like the paper under the ramp. I realized later that I should have had the kids measure the starting height of the ramp. Then we could go back to it later when we do energy and analyze the results.
Next year I think I will have them hit the penny first time, then a dime, then a small washer, so each time it must be more accurate.
As always, comments and ideas are welcome.
I now feel qualified to put something up on this post. We played for two whole days in my physical science class, and the kids still want more. Another day in my three physics classes, and I’m battle ready.
I began by showing the balloon on the bald teacher’s head and sticking it to the wall. Trust me, nothing gets them going more than a mostly bald teacher trying to rub a balloon on his hair. Amazingly, there were quite a few students that had never seen a balloon charged up and stuck to a wall.
We then went to the standard acrylic/fur type of static charge, explaining how the charges separate. I caused paper and bits of styrofoam to jump from the charge.
The van de graaff generator is exactly like those static creating devices, but it just keeps making more and more static. Here are a few ideas I have either done or picked up on the internet. One important note; I got all of the kids up and involved. Some of them were scared, but after the girls charged up their hair without pain, the chickens were shamed into bravery.
Before doing each of these demonstrations, I ask the students what they think will happen:
1. I take a bunch of holes from a paper punch and put them on top of the dome. Then I turn the machine on and the holes fly up into the air. The dome and paper, all having the same charge, repel each other. The paper holes spray up in a fountain of white dots.
2. I tape strips of paper to the dome. The paper stands up and stays standing until the dome is discharged. This is a good precursor to the hair thing. They don’t expect the paper to stay up in the air when the machine is off.
3. I use the grounding electrode to make the sparks jump really far. Let the generator run until you hear the ozone crackling. Then you get a great big spark. I use this to build some tension and fear of the generator because I’m asking for volunteers to do the hair thing.
4. Making hair stand up. The student needs to stand on a plastic milk crate or something to insulate them from the floor. One student wanted to try this standing on the ground. I think he had sweaty feet, he said (and we heard) the discharge going through his feet into the floor. I wish I could tell you how to know what kind of hair works best. Really long hair is too heavy, really short hair is too stiff. Hair color doesn’t seem to matter, although dark is easier to see than blond. For some reason, the hair of the black girls worked best. I’d love to post the pictures, but posting pictures of student’s is a no-no, at least without written permission.
5. Fluorescent light bulb lights up. It does not need to come into contact with the dome, the spark jumping to the glass with light up the bulb. We found that placing the bulb about 1 inch from the dome gave the best results. Stand on a piece of wood or you will feel the shocks in your toes.
6. We made a chain starting with one person charged up. He touched the next person, but held on. Now they both charged up and continued to another person. If the person getting shocked was sitting in one of our desk/chair units, he or she got a constant stream of shocks to the legs and back side.
7. Water bottle on top produces lightening like show. I’m going to tell you to be careful with this one. It works pretty well at first, but the massive sparking in the bottle actually burned through the bottom of the plastic bottle. Once they started leaking, they wouldn’t charge up. I had to use a different bottle for each class. More importantly, the bottle kept the charge. Just holding the bottle and moving it around gave a constant stream of rather painful shocks. At one point I was holding the grounding rod and the bottle. I touched where the bottle was leaking through the bottom and I got an extremely nasty jolt across one arm to the other. Be careful with this one.
8. A balloon placed near the dome is first attracted, then when it touches the dome, the charge is conducted and it is repelled. The charge leaks off and this repeats over and over again. I used this to lead into Coulomb’s Law and the force due to the electric charge. Again, you will want to stand on something to insulate you or you will have toe sparks.
Here are a few demonstrations that I haven’t yet tried:
- Mini pie tins stacked on top fly away one at a time – the pie tins I tried were too big.
- Soap bubbles are repelled as they get near the dome.
Long story short, we went to Penn State, visited the nuclear reactor, toured the school, ate lunch, and purchased two coolers full of ice cream. It’s a 3+ hour ride home, so we also purchased dry ice for $0.75 per pound.
The next day, there was still quite a bit of dry ice left over, so I did this demo for each of my classes. I put the dry ice in water and watched the “smoke” pour out. I had another beaker with water and some dish soap. Adding dry ice to that makes a huge pile of bubbles that are cloudy and evaporate on contact.
The best part was that the kids suddenly had a ton of questions. They wanted to know what would happen if they inhaled the gas, if the water was safe after the dry ice was gone, and lots more. Don’t forget to play, it’s a great way to learn.
This is one of my favorite simple demonstrations. I have a plastic spool that came from a pack of rope lights. The spool is about a foot across and I use it regularly to demonstrate a constant horizontal velocity.
When we study torque, I attach a pink mason line to the spool and wind it up. The question for the kids is, “What happens when I pull on the string?” Now, if it is unwinding from the top, obviously the spool is going to go in that direction. In the case of the drawing on the right, the spool is going to roll to the right.
But what happens when the spool unwinds from the bottom? Most of the kids think it is going to move to the left. A few will guess that it will stay in place, unwinding and slipping at the same time. Only a few think it will move to the right.
It does in fact move to the right. If we consider the point at which the spool meets the table our fulcrum, then we have a torque causing clockwise rotation according to the drawing at the right. The force is going to wind the string around the spool. Don’t believe me, try it for yourself.
(This was submitted by Duane, a High School teacher in Georgia. Thank you Duane.)
One fun “observation vs. conclusion / assumption” demo that I love came from Flinn Scientific’s “A Demo A Day” for Chemistry. I call it the “Potato Candle”.
Cut a cylindrical core (apple corers work well) from a potato – rinse it in lemon juice to preserve the near-white color of the cut potato – then cut a cross in one end. Insert an almond sliver (available at any grocer in the baking goods aisle) into the sliver. Your “candle” is now ready for the discussion / demo.
Inform your students that they are to practice their powers of observation, and make as many observations about what they are about to see in a limited time frame. Turning the lights down or out aids in their “mis-observations.”
Light the almond sliver with a match – it will catch readily, and burn for about 2 minutes – so don’t give them much longer than 60 seconds to make their observations. Blow out the almond before it burns out, turn on the lights, and start taking notes on the board as to the observations the students made of the “candle.”
At some point, particularly effective after someone makes the observation that the “candle” is made of wax, note that that’s an interesting observation, calmly bite the potato candle in half, chew and swallow. Your students will be aghast for a moment, wonder if you’re as crazy as that seems, and it leads into a lively discussion on the differences between observations, conclusions, and assumptions based on previous experiences.
Hope you like it!
There is no easy way to demonstrate this in writing, so I will be brief. (Maybe I’ll video this at the gym tomorrow night and post it here.)
A key to martial arts is using an attackers momentum against himself. We don’t want to use direct force against an attack when a small redirecting force creates so much more havoc.
The move I have in mind is when an attacker is swinging a sucker punch or roundhouse punch. By blocking and pulling on both the punching arm and neck of the attacker, you can send them down to the floor. ( I don’t recommend this to those without some serious martial arts training.)
The physics here is all about impulse. My effort is minimal, I use a very small force to add to their momentum and throw the attacker off balance.
Yeah, like I said, I need to make a video of this one.
I realized tonight, as I start to plan my lessons for the week, that I don’t have much here on momentum. This is a pretty straight-forward section. It’s easy to teach and should not be confusing to students that do the barest amount of studying.
As a quick summary, I teach momentum and impulse, skip angular momentum, then teach conservation of momentum. Not all the books discuss elastic and inelastic collisions, but I think that is rather critical to the subject. I also stop short of including the energy section.
Once I start conservation of momentum, I talk about what happens when people don’t wear seatbelts. I teach high school seniors in Philadelphia, many of them are just getting their license because they use public transportation to get around. I drum into their head the need for seatbelts. One way to do this is to figure that a car traveling at 25 m/s (about 55mph) hitting a tree would compress perhaps 1.5 meters while coming to a stop. I calculate the time to stop is about .12 seconds. Using the impulse-momentum theorem, I get a figure of about 300,000 Newtons of force. I’m sure car manufacturers have better numbers, these are just an estimate.
For demos, I have something call “slippery alley” that was purchased through Frey. There is a sled that wants to separate because of a rubber band, but it is held together with a string. The alley is a metal trough filled with plastic microspheres creating a nearly frictionless surface. Since one part of the sled is twice the mass of the other, when they seperate (in opposite directions), the lighter mass is traveling twice as fast as the heavy one.
The worksheet I’ve attached was very carefully researched. All the masses are accurate, as are the velocities of the projectiles. If you find any errors, please let me know.
This is an interactive applet that allows the student or teacher to change the velocity and acceleration of a car. There is a graphical representation of the car and the velocity vector as it moves across the top of the screen. On the bottom are three graphs that show the distance v time, the velocity v time, and the acceleration v time. I wish I had found this a month ago, it would have been helpful explaining these principles.
Today my Physical Science classes did two short labs in one period. Both of these mini-labs came from the book “Super Science with Simple Stuff!” by Susan Popelka. The book is geared towards middle school, but that never bothers me.
The first was using air pressure to crush a soda can. I was going to do this as a demonstration, but when I tried it this morning, the impact of the event was so powerful, I decided to have the kids do it themselves.
What you do is take a soda can and put about 1/4″ of water in it. Heat the can over a bunsen burner until the water is boiling. If you have a triangle support on a ring stand, it takes about one minute. If you have an asbestos wire mesh, it takes a couple more minutes. When the water in the can is boiling, use tongs and invert the can into a bowl of water. The can implodes instantly and dramatically. I’m a jaded science geek and it impressed me. The kids absolutely loved it. I had extra cans so they could do it again, they used up all my cans in both my classes.
The second lab uses Corn Syrup, Water, Vegetable Oil, and Rubbing Alcohol. I used a 250ml beaker and had them put 50ml of each liquid. First the corn syrup, then the water. Before adding the water, they added a drop of blue food coloring. Next they added the oil, but poured it over the back of a spoon so it would cause the layers to mix. Last the alcohol with a drop of red food coloring, again poured in over the back of a spoon. You get four very distinct layers. Then use random items to see if they float in between the layers. I used wooden toothpicks, bits of a plastic spoon, beans, and bits of Styrofoam from a cup.
The kids all commented how cool the lab was today. They enjoyed it and were really excited.