FizziksGuy

2,773

48

1. Ion engine could one day power 39-day trips to Mars

Very cool... now if only we could get the space program to undertake an ambitious goal and really push technology again!!!
2. castle learning

I'm betting this is the problem to which you're referring... A 1.00-kilogram mass was dropped from rest from a height of 25.0 meters above Earth’s surface. The speed of the mass was determined at 5.0-meter 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!
3. castle learning

Can you give us more detail? Problem #? Cut and paste the problem here? Thanks!
4. SmartPhone app for APlusPhysics.com

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. :-)
5. Help with Gravitation WebAssign!

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!
6. Rotational Motion Challenge: Win a \$20 Starbucks Gift Card!

[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
7. Input Needed: Direction of Angular Velocity and Angular Acceleration Vectors

[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 step-by-step 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!
8. Input Needed: Direction of Angular Velocity and Angular Acceleration Vectors

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 right-hand-rule, 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?
9. Help with Gravitation WebAssign!

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!
10. Help with Gravitation WebAssign!

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...
11. Unrolling Toilet Paper

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.
12. Physics in a Winter Wonderland

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.
13. Want to Publicize Your Blog? List it on Edublogs!

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!
14. 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!
15. Lunar Eclipse Tonight... don't miss it!

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.
16. Lunar Eclipse Tonight... don't miss it!

Too cloudy to see anything yet... hopefully will clear up in a few hours! More details here: http://is.gd/j1AaA

18. The Fugitive: Conservation of Momentum and Free Fall Kinematics

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
19. Rotational Motion Test-Taking Tips

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!
20. Anti-Gravity

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!
21. Physics is everywhere in life!

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!