Archive for the ‘Demonstrations’ Category
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.