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Everything posted by FizziksGuy

  1. In his Dec. 17 Action-Reaction blog post titled "Falling Rolls," one of my heroes of physics instruction, Frank Noschese, details an exercise from Robert Ehrlich's book Why Toast Lands Jelly-Side Down. The exercise, a rotational motion problem that challenges students to find the ratio of heights at which you can drop two identical toilet paper rolls, one dropped regularly, the other dropped by holding onto the end of the paper and letting it unroll, such that the two rolls hit the ground at the same time. It's a terrific, easy-to-replicate and demonstrate problem that pulls together a great number of rotational motion skills --> finding the moment of inertia, applying the parallel-axis theorem, identifying forces and torques from free body diagrams, and converting angular acceleration to linear acceleration. My students dove into the challenge with zest! To begin the exercise, we set our variables (H=height for dropped roll, h=height for unrolled roll, r = inner diameter, R = outer diameter), then identified the time it takes for the dropped roll to hit the ground using standard kinematics: Next, we did the same thing for the unrolling toilet paper roll: Of course, if we want them to hit at the same time, the times must be equal, therefore we can show: Obviously, what we really need to focus our efforts on is finding the linear acceleration of the unrolling roll. To save ourselves some time, we started by looking up the moment of inertia for a cylinder: Using the parallel-axis theorem to account for the unrolled roll rotating about its outer radius we find: Next, we can use a free body diagram to identify the net torque on the roll as MgR, and use Newton's 2nd Law for Rotational Motion to find the angular acceleration: Since linear acceleration can be found from angular acceleration multiplied by the radius of rotation ®: Finally, since we're looking for the ratio of the dropped height to the unrolled height: This conflicts with the results from Noschese's class, where they derived However, their demonstration based on their results is very convincing. Let's take a look at the difference in ratios using the two derivations: For a toilet paper roll of inner diameter .0095m and outer diameter R=.035m (our school rolls from the janitor supply closet): It appears that our derivation is correct, per our visual confirmation with a high speed video camera: You can follow the original blog response at Physics In Flux.
  2. Over the river and through the words, to grandmother's house we go... the horse knows the way to carry the sleigh through the white and drifting snow - oh! [ATTACH=CONFIG]67[/ATTACH] As part of our family's holiday season festivities, we went on a horse-drawn sleigh ride through the woods in northwest Pennsylvania. It was a terrific time, with low winds, just a very light dusting of now coming down, and 28 degree temperatures. As Miss Micro-APlusPhysics (aged 16 months) drove the sleigh, I couldn't help but think what a terrific multi-faceted physics problem our trip would make... finding the force of friction the horses had to overcome to keep us moving at a constant velocity through the woods, the power supplied, and the energy consumed. Of course, being a physics teacher, I couldn't just leave it there: With nine people on the sleigh, all bundled up, I think we can estimate an average mass of about 70 kg per person (we had a couple lightweights, including the baby.) So, the mass on the sleigh was probably on the order of 650kg. The sleigh itself was made out of fairly solid boards with steel runners, and a quick attempt at lifting up a corner provided a feel for its weight -- let's estimate the sleigh at 550kg, giving us a total load of 1200kg. The weight of the load, then, settles in a 12,000N. The horses pulled the sleigh from a horizontal tether, so that given the equilibrium condition of the sleigh, we know the normal force had to offset the weight, so the normal force of the snow on the sleigh is 12,000N. Now, to estimate the coefficient of friction. From the NY Physics Regents Reference Table, we find the coefficient of kinetic friction for a waxed ski on snow as 0.05. This seems like a reasonable esimate for the frozen runner on the snow. Using we find the force of friction as 600N. For most of the 20-minute (1200s) journey the horses pulled us at a leisurely constant speed of approximately 1.5 m/s. Therefore, we can assume the applied force of the two LARGE Belgian horses as 600N. The power supplied can be calculated from P=Fv, or (600N)*(1.5 m/s) = 900W. And since they applied that power for roughly 1200s, the work done by the horses can be found from W=P*t=(900W)(1200s)=1,080,000 Joules, or the equivalent of 258 food calories (roughly the nutritional equivalent of one slice of pizza)! A fun holiday activity providing another opportunity to highlight physics in the world around us.
  3. If you're interested in publicizing your blogs outside just the APlusPhysics community, and perhaps put yourself up for a Blog of the Year award in 2011, consider listing your blog at Edublogs. You can click here to submit your blog for inclusion in their directory of educational blogs. You can find your RSS feed by clicking on the small orange RSS button on the top right of your blog!
  4. FizziksGuy

    What causes gravity?

    I'm still a fan of the femto-elves with pointed hats taking tethers and pulling objects with mass toward each other... but assuming that ISN'T really the case, there's a lot to be said for Einstein's general relativity making intuitive sense -- no supporting data, but it does seem to make intuitive sense and relate well to our understandings of the universe. Now, having said that, my giant beef with string theory is the fact that there's no supporting evidence, yet it's become extremely popular because "the math works out well." Hypocritical? Absolutely... so we keep searching for answers!
  5. I'm not setting the alarm, but something tells me between dog and my micro-physicist something will have me up in the wee hours.
  6. Too cloudy to see anything yet... hopefully will clear up in a few hours! More details here: http://is.gd/j1AaA
  7. From 1993's "The Fugitive" starring Harrison Ford and Tommy Lee Jones: http://www.youtube.com/watch?v=RLbG3kxQ8dI&safety_mode=true&persist_safety_mode=1
  8. Some excellent points... and you're absolutely right, in a majority of the problems we've worked through from old AP exams, our "bridge" equations were the translation to rotational kinematic equivalents!
  9. FizziksGuy


    So, masses attract each other... opposite charges attract each other... opposite poles attract each other. We can have individual masses. We can find individual opposite charges. Have we ever found an individual magnetic pole (a singular north or a singular south pole)? If you find a magnetic "monopole," take very good care of it, write a paper about it, and send me a picture when you win your Nobel prize!
  10. What a wonderful Christmas present to read your post... Finding joy in learning is a blessing in life, and it's never going to stop!
  11. A colleague and respected writer from the physics blogosphere asked me this morning if I could explain what APlusPhysics is all about, and why it's worth the effort. Wanting to build up the APlusPhysics community, of course I jumped on the opportunity to distribute information about the project, especially to someone who has a significant following on her blog -- we can use all the targeted advertising we can get! I had many convoluted answers to the request, but realized I hadn't truly put them together into a "big picture" view of my vision and goals for the site. So, with the help of several friends who graciously offered their critical thinking and editing skills, I believe I have a reasonably complete answer to her question. My response, which I also posted on my personal A+ Physics blog, may be of general interest to readers here, so I'm including it below: My goal with APlusPhysics is to create a friendly, coherent and dynamic online resource with a consistent theme; an integrated toolset which can be easily customized to meet the needs of a diverse student and educator constituency while incorporating best known practices in physics education research. The site is designed for easy integration with physics modeling strategies, standards based grading (SBG), mastery learning, and “alternate pathway” programs which support students who, for various reasons, aren’t able to fit into the standard classroom educational model. It’s a work in progress. I’m learning as I go, refining, expanding, deleting and rebuilding. And then doing it all over again. I’m thankful for the support of the physics community as they provide feedback, ideas, opportunities, and constructive criticism that allow for continual refinement and growth from a variety of perspectives, and whose thoughts and ideas are the foundation of this online conglomeration. I hope you find APlusPhysics a useful web resource, and this blog an insightful journal of a developing teacher’s successes, failures, challenges, struggles, and achievements. Welcome one and all! WHO AM I? I’m a high school physics teacher learning something new every day. I was an engineer in industry for more than 10 years, and an adjunct college professor for eight, yet after three years teaching standard introductory (Regents) as well as AP-B and AP-C physics classes, it is obvious to me that student learning styles are changing rapidly… the standard “by-the-book” pedagogy is no longer the optimal method for teaching all students. I need to find a way to differentiate across a wide range of abilities, interests, backgrounds and habits if I want to help each of my students grow to their maximum potential in the brief time I have with them. I don’t have all (or many of) the answers — I don’t even have all the questions! What I do have is the energy and ability to learn, make changes, take risks, succeed, fail, and ultimately, grow. This blog details my journey. WHY GO TO THE TROUBLE? Writing is thinking. Writing forces you to organize your thoughts, to make mental connections — analyzing what’s worked, and what hasn’t. It forces you to think through your next steps, to reflect on why your experiments succeed and fail. It helps to recognize what you do and don’t know, providing a well-lighted path toward “filling in the gaps.” No single text or resource completely matches the way you teach. Our class text is a wonderful resource for our students, and I was even lucky enough to serve on the committee which selected the book during my second year in the classroom. It’s accurate and thorough. It aligns nicely to our district outcomes and state standards. But it’s not designed specifically to the course I teach and the method in which I teach it. Further, students are reluctant to learn and read independently from our text. This is troubling. The most important skill I can teach my students before they leave my classroom and go on to bigger and better things is the ability to teach themselves. Empowering them as learners requires technical reading, critical thinking, and discipline. I struggle with this throughout the entire year, and each year set a goal to extend my students’ independent learning skills through guided inquiry, discovery, and practice. Still, though, in many cases, even with our text, there are gaps. BRIDGING THE GAPS I have embarked on a project to create my own online physics resource, tailored specifically to course objectives, with as little extraneous information as possible, and consistent with the methods and organization I use in my classroom. I’m learning and changing every day, so this resource has to be dynamic. Problem solving practice with immediate and constructive feedback should be integrated into every unit. Most importantly, students should learn at their own pace. With a tremendous span of abilities, backgrounds, and learning styles, it’s obvious that one size and speed doesn’t fit all. Key aspects of this resource, APlusPhysics, include online discussion forums promoting discourse about concepts, applications, and new developments in science; online homework help where students can assist each other (the best way to learn is to teach!); student and educator blogs for learning logs and self reflection; course content distilled down to the “need-to-know” facts with a variety of sample problems, designed specifically to meet course objectives; built-in quizzes to allow students to test their understanding; and resources for physics instructors focusing on student-centered active learning activities. Many of these resources can be found, in whole or in part, elsewhere on the web. The Physics Classroom is a terrific online resource covering a wide variety of topics in physics; Cramster is a terrific resource for homework help and problem solving; Physics Forums is a terrific bulletin board system discussing physics developments and problems; Castle Learning offers students a tremendous repository for problem solving practice; and of course there are many others. I’m not trying to rebuild or re-create any of these terrific resources… they all have tremendous potential for the students who take the time to learn and use them productively. However, the learning curve for this expanse of resources can seem insurmountable to the new physics student already exhibiting the classic “deer-in-headlights” shock I’m sure all physics teachers are familiar with. This project is an ongoing method of delivering, refining, and reflecting upon high school physics education.
  12. Scientists have been measuring the universal gravitational constant, G, for hundreds of years. But, how accurate are they in their measurements? Is G truly a constant? It’s a question physicists and astronomers continue to debate. Due to variations in experimentally obtained values for G, a number of postulates have been proposed which note that G may vary with time, and could be dependent on orientation, surrounding masses, even the curvature of space time! Complicating matters, experimental error in the determination of G is typically estimated at 1%, even with modern measuring equipment. Is G really a constant? Does it vary within this +/- 1% window? Is Newton’s Law of Universal Gravitation complete, or is there more to it? Recent studies continue to explore and debate these questions. [ATTACH=CONFIG]64[/ATTACH]
  13. Dr. Jim Kakalios from the University of Minnesota does a tremendous job talking about the relationship of momentum, impulse, force and time in the context of a Superman comic book. If you want to learn more about Dr. Kakalios, he was highlighted in a three-part interview last year as part of the Physics In Action podcast. You can download these and other past episodes from the APlusPhysics Podcast Page. His book, The Physics of Superheroes, is also a terrific introductory-level book that examines physics from the standpoint of a variety of comic book superheroes. I highly recommend it, and have also had a number of students read it with 100% positive reviews.
  14. FizziksGuy

    The physics of baseball

    I've also seen the sweet spot modeled by looking at the nodes and antinodes in a bat http://www.physics.usyd.edu.au/~cross/baseball.html. You can also find the Physics of Baseball as part of a freshman physics course here: https://teamphysics.physics.uiuc.edu/SiteDirectory/PHYS199BBBlog/default.aspx And of course, KQED Quest's Physics of Baseball videos, which we often show in class... http://www.kqed.org/quest/television/out-of-the-park-the-physics-of-baseball
  15. FizziksGuy


    How do high jumpers use center of mass to maximize the height they can clear?
  16. FizziksGuy

    Motion , Impulse, Momentum

    Sounds like a good start... might be worth reviewing what motion is just to be clear moving forward. We'll keep practicing so these make more and more sense!
  17. Exactly... T1 as in the tension due to mass 1, and T2 is the tension due to mass 2. Also, don't forget that you need to use 9.8 for g in the WebAssign problems.
  18. Hello NSPhysics, and welcome to the APlusPhysics Forums! Please make yourself at home, and if you have any questions or requests, just let me know!

  19. Keep in mind that you have two torques still on the wheel, so: . Combine that with Newton's 2nd Law equations for both mass 1 and mass 2 and you'll have a system of equations you can solve to obtain your answer. :banghead)
  20. APlusPhysics Blogs and Forums have now been integrated with Facebook! Not only can you link your accounts, but you can simultaneously post any forum messages, blog entries, and comments to your Facebook wall by checking the "Publish to Facebook" checkbox at the bottom of your entry. Further, you also have the ability to "Like" blogs and forum posts... I'm interested to see if this encourages discourse and engagement by integrating a student favorite social networking site with classroom content.
  21. Wish I could re-design the tongue channel on a couple dogs I know...
  22. APlusPhysics forums and blogs have now been integrated through Facebook Connect! This means you can share your blog posts and comments, forum posts and comments, and "Like" button information on your facebook account. Check it out by clicking on the "Facebook Connect" button on the top of the page. If you want to share your posts on facebook, make sure to check the "Publish to Facebook" checkbox before hitting submit.
  23. That's quite a unique perspective... I wonder what other everyday phenomena we could look at in a whole new light by changing our frame of reference?
  24. FizziksGuy

    Rotational Motion

    Software (cookie) settings have been updated to allow you an hour and 20 minutes before you're automatically logged off in the future. Hopefully this will prevent you having the same frustration again.

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