Posts Tagged ‘Newton’s Laws’
We finished the lab today. I gave the kids two days to do it. Most of them figured out the initial velocity by the end of the first day. The start of the second day, I put two hints on the board. For question 2, I put up t=d/Vagv. For question 3, I told them they needed to calculate the acceleration of the popper.
I decided to be only somewhat helpful. At the start of day 2, I told them the initial velocity should be in the range of 5 m/s. I told them I would not answer questions about their numbers if the formulas were not there and units were not shown. I generally only told them they were either on the right track or wrong, nothing more. Most of them had a tough time making the leap to the distance in part 2 was how far the inverted popper moved from rest to the calculated initial velocity. Once they got that, they were well on their way to solving the problem.
I did an interesting experiment while they worked. I set up a LabQuest to sample at 1 ms intervals. I build a tiny tray from cardboard and string and attached it to the force sensor. I set the meter to trigger at a force greater than 2.5 N, zeroed the sensor, and let it rip. It showed a nice impulse function that took 23 ms and a peak force of close to 7 N.
I could use some help with my interpretation of the graph. I believe the integral of the Force v. Time curve gives me the impulse (the LabQuest gave me a value of 47 N*ms). If I divide that value by the mass of the popper (9.1 g), I get a delta v of 5.16 m/s. This is in agreement with the numbers the kids got in the experiment.
Now if I divide the delta v by the time, I should have the acceleration. The LabQuest samples every millisecond and there are 23 points, so I think the time is either 22 ms or 23 ms. The acceleration works out to be 235 m/s^2. Doing this, I only get a force of 2.1 N, but the graph shows close to 7 N. The students calculated forces in the 6-7 N range. I think the discrepancy has to do with using the integral (which should be more accurate) and getting a peak force compared to an average force. Can someone either confirm this or correct it for me please?
Am I the only teacher that spent half of the holiday break grading papers and working on lessons? Here is a lab my students will be working on when they come back from break on Monday. I figure it will give me a day or two to settle in without having to get up front and teach.
We just finished Newton’s Laws before break, what better way to refresh their memory than making them think. I got this lab from the NSTA regional conference in Baltimore, it is called “Inquiry in a Box” and presented by Deborah Roudebush. I put the instructions into a format my students are more familiar with and I expect they will need two days to get their arms around the whole thing. What is very different about this lab (compliments to Deborah) is that the students are given only the problem to solve, some minimal tools, and no instructions. They need to figure it all out on their own. It could be a disaster, I fully expect a lot of whining.
The basic idea is that the half ball Party Popper shown above is a cool little science experiment. Giving them only a ruler and access to a gram scale, they need to figure out how to determine the velocity, time, and force exerted by the popping event. At the conference, we were put into groups of four and set about solving the problems. It didn’t take us too long, but there were some very good discussions on when the time and acceleration actually occurs. There will be no answers posted here, some of my students know about this site. If you need some help, email me.
Here is the lab handout: Popper Lab Handout
Now, you would think these little poppers are easy to come by… good luck! I went to many toy stores and party stores and found none. I ended up online at Oriental Trading Company. Their 1.5″ poppers are great, their 0.75″ are going back, they don’t work at all. I found another place selling them; Century Novelty. I’m ordering 1″ poppers from them. The key here is you have to plan ahead for this lab, you can’t run out to the store the day before and find them easily. I won’t have the 1″ poppers in time for this year, but next year I plan for them to analyze different size poppers and compare the results.
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 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.
When we discuss normal forces, I drum into the students’ heads that the force is perpendicular to the surface. They get that eventually. Where they get into trouble is on an incline when the angled component of the weight of an object F(i)=mg*cos(Θ). I called this force F(i) meaning the force exerted on the incline (and the (i) is really a subscript, but I can’t make that work in this blog).
This is sometimes the normal force, but not always. If there are any other forces, like F(x), in the angled Y-axis, then the normal force is not the same as F(i).
Here is what I tell them: imagine there is a scale under the object in question, what would it read? If it is only the object and no other forces, then F(n) is F(i). From the drawing above, it’s fairly clear that there are two forces down that combine to create our normal force up. The scale under the block would read the value of F(i) + F(x), so that is our F(n).
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.