Posts Tagged ‘Momentum’
First, let me be completely up front. I borrowed this activity from my pal Deborah Carder. You can find her link in my blogroll, she does great hands-on activities and labs. I met her at NSTA Philly last year, she is the Energizer Bunny of science teachers, I don’t know how she does it.
Anyway, as I had mentioned in earlier posts on momentum, I wanted to do the egg drop competition, but I’m in a one-story building. This year we are doing the “Egg Crash” competition. The basic concept is that teams get 10 sheets of computer paper, 1 meter of masking tape, a pair of scissors, and 20 minutes to construct a free-standing object to safely catch the egg. They drop their own egg from a height starting at 0.5 meters above the top of their structure. The egg is inspected before and after each drop, the higher they go, the more points they win. The surviving eggs are dropped from 0.5 meters higher each round until they all finally break.
I usually allot 25 points for a lab, I will probably go 50 for this one. Deb said she does 100 points, but that’s a test grade and I don’t think a one-day lab should be worth that much. I’m still working out the scoring, but I think I will assign a grade to each height. If they fail at 0.5 m above their structure, they get 30/50, but they also get a single start-over with a new egg. Each 0.5 m interval earns 5 more points. That means surviving 2.0 m earns 50/50. I’m willing to give 5 or 10 extra credit points if they can survive a drop from 2.5 or 3.o meters over the top. I was going to do direct competition for the points, but what if everyone fails at 1.0 meters? With my grading system, they all get 70% since nobody really earned the A or B.
I handed out the Egg Crash Description and Rules on the day we started learning about Momentum. I told them it will be about a week before we do the competition; I wanted them thinking about the problem and their design as we learn about momentum and impulse. This week I will show a great video called “Car Crash Tech.” The video discusses the state of the art auto safety systems and the effects of air bags and other innovations.
I’m hoping for some creative solutions from the kids. Maybe I’ll have a picture or two to post here in a couple of weeks.
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.
My wife asked me why my Rollerblades were in my trunk. She knows I don’t use them where we live, there are too many hills. I’m a Physics teacher, the answer should be obvious – Conservation of Momentum. Tell me the truth, don’t you look to do something unexpected to get you students’ attention. In graduate school we called it disequilibrium.
You should have seen their faces the day I skated into class. Of course I have the racing bearings which work well for classroom demonstrations. The idea is to start at a standstill and throw a heavy object and watch the object go one way and me the other. I also carried a box of textbooks and while moving forward, threw them ahead of me. They could see that I instantly slowed down. This is what dreams are made of. There will be NO photos, use your imagination.
I haven’t done this project in a couple of years. The reason is simple, our school is in a one story building. There isn’t any place near enough to drop these things. I wanted to do it again this year, I need a drop zone. I was considering building an air powered launcher or maybe a giant sling shot. Maybe a large catapult. I’m still thinking.
If you haven’t done an egg drop project, you need to try it at least once. You also need to really research some of the college competitions. The rules are very specific and you have to watch out for creating loopholes. Parachutes are never allowed. There are volume limits and maximum dimensions. Obviously the eggs are raw. The project can get very messy.
I like to do these as competitions. The first time I did this, I was at a school that had two floors. I figured none would survive the two story drop. All but one did. We decided to get into vans and go to the top of a nearby five story parking garage. Rather than drop them one at a time, we just launched them all at once. Good thing too, security showed up. Yeah, we probably should have called and asked permission. Next time. Anyway, two of the eggs survived the five story drop. Who says physics is boring?