Posts Tagged ‘Impulse Function’
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.