My setup originally had an error. I made each string supporting the pendulums the same length, which made the heavier pendulum longer, since I was using a larger, heavier bob. The heavier pendulum was about 4 cm. longer than the lighter one. (See the first picture.) I performed two trials with these pendulums before I realized my error and corrected the lengths of the pendulums so that the total pendulum length was equal. (See the final two pictures.)
My pendulums were suspended from pot hooks in my kitchen (see the second picture), allowing a stable point from which they could swing.
I then put each pendulum through three trials. When I had done the “error” trials, I discovered that I had to make a decision as to when I would decide when the pendulums had come to rest. I determined for the sake of the experiment that this meant that the pendulum must remain entirely still, which took more time than expected. For the lighter pendulum, the average time for the pendulum to come entirely to rest was 303.3 seconds. For the heavier, the average was 615 seconds.
I did expect the heavier pendulum to take longer to come to rest, since it would have the greater momentum since it had the greater mass, but I did not expect the pendulums to take as long as they did to come to total rest.
Overall, the experiment went well, although it was difficult getting the pendulums started and also starting the stopwatch at the same time by myself. This will be less of a problem in the classroom since the students will be working in groups.
In order to get a modified result, one that I hope would result in a shorter time for the pendulums to come to rest; I would have them drop the pendulums from a lower height. I wanted to keep consistent throughout the experiment so stayed with a similar drop height. I also wondered if longer strings, resulting in a longer period of motion would also allow the pendulums to come to rest more quickly. I will have to try that out at home to see what happens.
I may use this experiment in my class when I teach force and motion to my eighth grade in the last quarter. Thinking about ways to make it more exciting include using “Jeopardy” style trivia on the topic, which they love, while they wait for the pendulums to stop (I’ll have to think of a way to be sure someone remains the spotter!). Super long pendulums hung from the ceiling with larger masses done as a large group activity would likely spark their interest too. I would also use the “Pirate Ship” animation displayed on my class smart board as a “bell ringer” activity. This activity can be related to familiar things such as playground swings and amusement park rides, so they can understand the principle has applications in everyday life.
This experiment helped me to understand more about momentum, and to be able to see it separated from surface friction. I want my students to understand about the air exerting friction on objects, and I can follow up with information about rocket ships and re-entry; and why heat shields are necessary.
Your photos are a great display of your experiment!
ReplyDeleteLooks like you turned your kitchen into a science classroom. I like how you quickly picked up on the fact that the heavier weight was weighing down the string. That really would have changed the results of your experiment if more than two variables were different at the same time. Close call!!
ReplyDeleteI performed the same experiment, but my results showed that the amount of time it took for the pendulum to come to rest was the same for all three weights used. See my data on my blog address. When I changed the length of the pendulum string, I received much different times. It seemed that the longer the string, the longer the time. Maybe I was measuring something different. It looks like I am going to have to try it agin! Great photos by the way!
ReplyDeleteAfter conducting the experiment on a much more controlled basis (actually hanging the pendulum rather than holding it with my hand), I found that the larger mass definitely swung for a longer period of time than the smaller one. I must have been subconsciously attempting to control the results because my mind believed at the time that since a period of oscillation is not supposed to be affected by mass, the amount of time it took to come to rest would not be changed. Wow, what was I thinking! The reason the pendulum ceases movement is because the air and the friction of the setup places a force that slows down the force of the moving pendulum. Without these forces, the pendulum would continue moving indefinitely. Logically it is going to take more time of a constant force applied to stop a heavier object from moving because it is moving with more force (F=ma). My application paper is filled with erroneous data. That will teach me to take the shortcut in setting up a lab. This would be a good example of something to share with students about the importance of using a mechanical setup over using human hands. Oh, such is science!
ReplyDeleteThat was the clearest case of the danger of preconceived notions in science I ever experinced!
DeleteThanks everyone for your comments.
ReplyDeleteI can only put my original error down to "daylight savings time syndrome", I actually do know better than that about pendulums. I was pretty annoyed at myself, just look at the expression on my face in the one picture!
Aaron, I was kind of concerned when I read your original blog post because to be frank, I thought I might have been doing something wrong. Having already made one error with a factor I knew made me lack confidence.
I re-read the material about momentum, and although nothing in our assigned texts directly addressed the mass of pendulums, I felt as if I was indeed on the right track.
Eileen, love your post. I also suffer from "daylight savings syndrome", or actually its just teacher needs a vacation syndrome!
ReplyDeleteLori
Eileen,
ReplyDeleteGreat post and discoveries. One can see the power of collaboration by making their way through the comments. Even teachers are making discoveries!! Our students do not realize we continue to educate ourselves and science is about participation and critical thinking-great example!