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Everything posted by FizziksGuy

Tremendous! I can't wait to talk my brotherinlaw into demonstrating how it works on his Android phone! And also impressed by the QR codes... one of my "todo" items includes integrating these codes with our course materials. Let us know what feedback you get!

I don't think you're alone... so far you've always known what unit you've been working in, but now that we've covered a typical college semester at a high level, having things jumbled together isn't uncommon, especially after a long break. The good news  we have one or two more days of material to cover, then we have a chance to sort everything out as we prepare for the midterm. Take this time as an opportunity to clarify and organize your thoughts, to figure out your strengths and weaknesses  then attack the weaknesses, and fortify the strengths. You know more than you think you do... our first WebAssign practice MC test is designed to be a tough one... an exam to help you realize where you need to work  but not in a comfortable way. Use it as it was intended  a tool to help you focus your efforts moving forward. And if it rattled you a bit, perhaps that's not a bad thing either  but don't let it throw you from your course!

Very, very cool! I am, yet again, impressed. Well done probablykevin!

Hiya Grace... yup, got this one too. Just didn't get home until late last night. I'll make sure to up your Castle Learning score by 1 point. Have a great weekend!

Amazing interactive flash demonstration: Scale of the Universe

A great video on supersonic flight from AVweb!

Great thermodynamics video!

Pretty amazing  and this will tie in very nicely with our upcoming study of springs and Hooke's Law!

No apologies required for String Guess... it's a very slick theory, and would be awfully nice if it were true  just hard to call it a theory without any evidence! So many things yet to discover  how many will we get to in our lifetimes?

Ion engine could one day power 39day trips to Mars
FizziksGuy commented on a blog entry in Blog awalts
Very cool... now if only we could get the space program to undertake an ambitious goal and really push technology again!!! 
I'm betting this is the problem to which you're referring... A 1.00kilogram mass was dropped from rest from a height of 25.0 meters above Earth’s surface. The speed of the mass was determined at 5.0meter intervals and recorded in the data table below. [ATTACH=CONFIG]72[/ATTACH] In this case, reading the velocity off the graph directly, I would estimate the speed of the mass as approximately 15.5 m/s (which Castle Learning should accept...) Unfortunately, it apparently only accepts 15.7 m/s as its exact answer. Your responses were 15 m/s and 15.5 m/s. As I can't adjust the assignment once it's been passed out, I will increase everyone's scores by 1 point on the assignment. Well done, and thanks for bringing this issue to light!

Can you give us more detail? Problem #? Cut and paste the problem here? Thanks!

Sounds pretty cool to me! Was just talking to Mrs. FizziksGuy this evening and discussing the probability of Amazon developing a tablet PC based on Android to compete with the iPad in 2011... I think it's going to happen, she just rolled her eyes at me. :)

10 and 11 are solved in similar fashions... start with conservation of energy: Then, of course, remember that: and You can find in your text that the radius of Earth is 6.37*10^6m, and with that information, each of those problems should become relatively straightforward. If you're still stuck, if you can post what you've done so far it will give us a better path to see what we can do to help!

Rotational Motion Challenge: Win a $20 Starbucks Gift Card!
FizziksGuy posted a blog entry in Physics in Flux
[ATTACH=CONFIG]68[/ATTACH] A colleague and friend of mine has offered a $20 Starbucks gift card to the student who can provide the simplest, clearest explanation of why the angular velocity and angular acceleration vectors point in the directions they do... check out the details and submit your entries in our Forums section! http://bit.ly/guQV0L 
Input Needed: Direction of Angular Velocity and Angular Acceleration Vectors
FizziksGuy replied to FizziksGuy's question in Homework Help
[ATTACH=CONFIG]69[/ATTACH]My colleague has offered to "kick up" the challenge a notch. Details below: The prize: a $20 Starbucks gift card. The challenge: Provide the best stepbystep explanation (ala a geometric proof) using only concrete, physical descriptors; in other words, NO MATH equations, detailing why the angular velocity and angular acceleration vectors point along the axis of rotation. Your audience: students currently taking a full year AP C mechanics course. Half of them are currently in Calc BC; the rest are in AB. Only 10 of the 30 students took AP B last year; the rest took Physics 1 or no physics before this current AP C course. Winner will be determined by a class vote from our "audience." Please post challenge responses below... enter as often as you like, but remember to keep your audience in mind as you write your explanations! 
Hi All, I had a friend and colleague ask me today about why the angular velocity and angular acceleration vectors point in directions given by the righthandrule, as highlighted here... When I read my response, I realized that my answer wasn't much better than that given in the link... I thought about it some more and started thinking that it probably related to: and since the cross product of r and F is perpendicular to both r and F, with the positive direction given by RHR, the angular acceleration (and similarly angular velocity) vector must be consistent. Still not a very pleasing or clear explanation. So, why not turn to the experts? If someone were to ask you, how would you explain the direction of the angular velocity and angular acceleration vectors in a manner that was as clear and straightforward as possible?

Hint for #3: volume of a sphere is: For this example, assume radius of planet is 4X Earth's radius... Start by finding mass of earth, which is density of earth * volume of earth: Mass of new planet must be: Using these two equations, solve for the mass of the planet as a function of the mass of Earth. Next, if you want the weight, or force of gravity, on this new planet: From here, you can substitute in for the mass of the planet and radius of the planet as a function of Earth's mass and radius to come up with a factor for the change in the force of gravity. As a final hint, for the case I provided (radius 4X larger), the force of gravity would be 4X greater. Good luck!

Hint: Find the mass of the new planet relative to the mass of the earth (how many times more massive is it?) Next, find how much the radius changes compared to Earth. Then, substitute these values into Newton's Law of Universal Gravitation to find the weight on the new planet. If you can provide some background into what you're doing, I'd be happy to take a look and see where things are going awry...

In his Dec. 17 ActionReaction 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 JellySide 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, easytoreplicate and demonstrate problem that pulls together a great number of rotational motion skills > finding the moment of inertia, applying the parallelaxis 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 parallelaxis 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.

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 horsedrawn 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 MicroAPlusPhysics (aged 16 months) drove the sleigh, I couldn't help but think what a terrific multifaceted 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 20minute (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.

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!

I'm still a fan of the femtoelves 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!

Lunar Eclipse Tonight... don't miss it!
FizziksGuy commented on FizziksGuy's blog entry in Physics in Flux
I'm not setting the alarm, but something tells me between dog and my microphysicist something will have me up in the wee hours. 
Lunar Eclipse Tonight... don't miss it!
FizziksGuy commented on FizziksGuy's blog entry in Physics in Flux
Too cloudy to see anything yet... hopefully will clear up in a few hours! More details here: http://is.gd/j1AaA
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