Posts Tagged ‘Inertia’
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.
This is a trick that most real woodworkers know. When you need to replace the handle of a tool, you put the head of the hammer on the handle and then you drive the head down by holding the hammer vertically and hitting the bottom of the handle on a concrete floor. This is also how you tighten the fit of the head on the handle.
Because the head is heavy, the downward inertia of the head causes it to continue down, causing it to tighten itself. I have an old hammer head in my shop that I’m going to purchase a handle for, just to demonstrate this principal.
Do I really have 517 demonstrations of inertia? It feels that way, but of course I don’t. I max out at around 300. Anyway, this is similar to putting an anvil on your stomach and then hitting an anvil with a sledge hammer. I’m just not that trusting. Here’s how I made it safe for High School seniors:
I take a box of books. I have a case of Conceptual Physics books, they weigh about 80 lbs. I have a student come up to my demonstration table and lie down. I like to pick one of my more vocal (translate as annoying) but also smaller in frame kids. They lay on their back and I gently place the box of books on their chest and stomach. When they agree they are not in pain, I usually call up the biggest and strongest kid in the class. His job is to slam his fists down on the box of books as hard as he can. This is usually a friend of the one laying down, so the drama is even better.
As hard as he hits the box, the kid on the table barely feels anything. While he’s still there, I ask them about removing some of the books from the box and what do they think would happen. I like for them to take the thought experiment to the extreme, so I ask if we take all the books away and replace it with a single piece of paper. Then what would happen? My hope is that they see how the large mass has a lot of inertia and resists a change in motion.
This usually moves to about half the class trying this on each other at my table. Chaos, you bet, I encourage it. They are interested enough to get out of their chairs and try an experiment, you better believe I’m going to let them. I’ll take every teachable moment I can get.
The string at “A” is tied to a solid surface. In my room, it’s the TV arm on the wall. String “B” is the same type of string, I usually set it up with two “B” strings hanging down. In the middle is a piece of wood with two eyescrews attached. Around the wood is a pound of dollar store clay. (I really hate dollar store clay, the dyes come out and stain everything it touches. I just don’t touch the clay and this setup goes back in a bag to be used one day a year.)
First, ask the students what happens if you yank quickly on one of the “B” strings. Allow some time for discussion. Inertia of the blob of clay is going to resist the motion of the yanking and the “B” string is going to break. Do it for them.
Part two, tell them you are now going to gently pull on string “B” and ask what is going to happen. In this case, you are slowly applying a force downward. Let’s say you pull down with a force of 10 pounds. String “B” feels 10 lbs of force, but due to the weight of the clay blob, string “A” should feel 11 pounds of force. Yeah, I know, kg, not pounds, but since clay comes by the pound, I wrote in pounds. Sue me. Anyway, string “A” is going to break first because it always has one more pound of force on it.
For the very first time this year, the “B” string broke for both of them in one of my classes. That sure took the wind out of my sails. I cut another piece of string and did it again, although I lost the wow factor. Can’t win them all.
I started building model rockets with my students this year and I’m glad I did. Most of my students have never built or launched rockets before. A few did in eighth grade, I think maybe two or three did with their parents, but out of the 100 or so seniors that I teach, that’s was it. About 80% of my students are college bound and only a couple of them are going into science or engineering, so connecting a subject like physics to something hands-on is critical to their understanding of the material. Not that I think most of them understand it, but let me delude myself please.
The school purchased one rocket for every two students. I know in some area schools, the students are required to purchase the materials. I know that most of mine could, but quite a few can’t. So the school paid for them. It took about three days to build and paint the rockets. They build, I paint. I knew I had to when one of my more trusted students came in with a rocket dripping paint. “Several light coats are better than one heavy coat.” Didn’t matter how many times I said that, apparently it didn’t stick.
The next nice day we all trudged out to the field, took lots of pictures posing with our rockets, then we launched them one at a time. I tried to explain how high and how fast they go, but until they saw it they just didn’t get it. A few dramatic failures are good. We had one tail fin fall off because it wasn’t glued on well. The rocket looped just barely over our heads. A few had the nose cone too tight. The ejector charge couldn’t pop off the nose cone, they come down fast and tend to stick in the mud. We even had one actually explode. I’ve never had that happen, I think it was an engine failure and not the work of the student. All these events add to the teachability of the lesson. We learn from our failures.
As a follow up homework assignment, they each had to write an article telling about the project, the launch, and explaining the theory to someone who hasn’t had physics. I chose two of the articles and a couple of photos and submitted it all to the school newspaper for publication.
- Each group got a single A engine with the rocket. If they wanted to launch again, they had to purchase an engine for $2. I had some B and C engines, but our field isn’t very big and we lost anything launched with C’s.
- I wanted the kids to purchase the rockets for $2, but only a few did. I would either get them to purchase them up front with their own money or just give it to them. The teams would have to decide ahead of time which of the two gets to keep the rocket.
- I bought a mix of Viking and Wizard rockets. Both are good, they use streamers for recovery rather than parachutes. A parachute in a 10 mph wind will drift twice as far as it is high. So if it goes up 500ft, it will drift 1000ft.
- Walmart is the cheapest place to find engines. A three-pack is under $5.
Have you seen one of these? It’s a small, battery powered hovercar in the shape of a large hockey puck. It’s only about six or eight inches across. When I’m teaching Newton’s 1st law, I take these out (I have three of them) and I launch them around the room. I let the kids play, kicking it around. Usually we take it into the hallway and see if we can make it all the way to the office from my room. After I get them back in their seat and on task, we review the concept of a inertia and the need for a force to change the puck’s directions.
I haven’t done this yet, but I’d like to show some video of a hovercraft and how difficult it is to make them turn. Actually, I’d love to build one someday. Something to keep in mind for a future lesson.
NOTE: The hoverpuck I purchased from the supply company uses a rechargeable battery. I don’t know how well it’s going to work next year. I just saw them at a local store called “$5 and Below” and it uses regular AA batteries. I think over the long haul, that is a better idea. I don’t expect the rechargeables to be working in a year or two.
These were new to me, I saw Lou Bloomberg do them at an Educator’s Day up at Penn State. This is for the teacher to do since it involves a sharp knife.
Place an apple on the edge of the table and use a long knife and a quick flick of the wrist to slice through the middle of the apple. The apple should stay in place because of its inertia.
The second part of the demo was even better. Lou held the knife in the air and threw the banana sideways at it. The banana traveled in an arc cut itself in two on the blade and kept on the same trajectory. Although expected, it was odd to see both halves continued to travel next to each other.
NOTE: Lou must have practiced this a million times. It sounds easy and he made it look easy, but damn, it ain’t easy. You might want to buy some bananas ahead of time and practice throwing a banana over your head.