Magic fence or power of inverse square?

On a walk through UC Berkeley, I came upon the following sign on a fence:

Close up of sign

Here’s a broader view of the sign and its surroundings:

Magnetic field fence

The distance from the building to the fence is about 2m (6′).

The question:

Has UCB discovered a new fence that can stop magnetic fields? Or, is this just an example of the inverse square relationship between the strength of a magnetic field and the distance from it?

Here’s the front of the building:

Wheeler Brain Imaging Center

The building houses the Henry H. Wheeler, Jr. Brain Imaging Center.

Here’s a photo of the imaging device:

If the distance from the magnets to the outside of the building is 1.0 m, what is the relative strength of the magnetic field outside the fence (an additional 2 m away)?

The physics (and biology) of snake movement

Science Friday had a great piece on snakes slithering today. The main researcher in the video is Dr. David Hu from the Applied Mathematics Laboratory, Courant Institute of Mathematical Sciences, New York University, New York, NY.

Turns out it’s much more compicated than one would think. The scientists used smooth surfaces and even Jello to test their hypotheses. Here’s the article about the video.

The math in a hybrid car

For an end of the semester project in my physics classes, I posed the following question to my students:

Students were asked to look up a car they would be interested in buying, but the car has to come in both hybrid and traditional engine models. They were asked to look at the price of the car and the cost of gas over the 100,000 mile “life” of the car (OK, some are going to sell it at 50K, and others hold onto it until it dies, but 10,000 seemed like a good average). We just finished a unit on energy, and I thought this would be a good way to get them thinking about energy and money.

If you want to know their results, you’ll have to keep reading, but I’ve since had time to think about making this question more open-ended, inquiry (thanks to Dy/Dan for keeping my on my inquiry toes). So now I’m thinking of the questions that could be raised in a class discussion:

• Write an equation for the cost of each car as a function of the miles driven.
• Using these two equations, solve for the intersection of the lines. What does this intersection represent?
• Car mileage is rated in both highway and city. Expand your equation to include a variable for the percent of driving that is city.
• How many extra fill-ups will it take to drive the traditional engine car? If your time is worth money, how much will this cost you if you make $50,000/year?
• How much gas is saved by driving the hybrid? What percent is this?

The list can go much further than this. Leave it up to your students to develop more.

Students did calculations from the Honda Civic to the Cadillac Escalade (yes, it comes in a hybrid). None of the hybrids broke even with the traditional engine in cost for 100,000 miles. Students were asked to write a short paragraph saying if they thought buying the hybrid was “worth it.” They gave great responses, including those who even looked at the gas they would save just from switching from the car they are now driving to a more fuel efficient car (one student said it wasn’t worth it to buy the hybrid, but certainly was worth it–financially and environmentally–to buy a new car to upgrade her mileage.

Interesting “ESP” site

The test

A friend sent me this. I was surprised at first, but then it didn’t take me too long to figure it out.

Here’s the test: http://sprott.physics.wisc.edu/Pickover/esp.html

Try it out (it will open in a new window), then return here if you can’t figure it out.

Scroll down for the solution.

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The solution

Hint: there is only one image for the “before” and only one for the “after.”

Scroll down for more details.

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Here’s the “before” image:

Here’s the “after” image:

Can you solve it now?

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Notice there are NO identical cards from the first set to the second.

Folks will focus so much on the card they want, that they don’t notice that none of the original cards are present.

Ball in water on elevator: Buoyancy

OK. Float a ball in a beaker of water in an elevator. The ball floats because the buoyant force of the water displaced by the ball is equal to the weight of the water.

Now, accelerate the elevator upward. What happens to the ball? Does it: Sink lower, rise higher, or stay the same?

Here’s my video with the answer:

Once the video is playing, you can click on it to get to the YouTube page with larger size and high definition versions.

10 Excellent web sites for teaching?

Scott McCleod posts: A seemingly simple question

New NorCal/Nevada American Association of Physics Teachers web site

One of my new tasks for 2009 is Web Weaver for the Northern California/Nevade American Association of Physics Teachers. Today we launched a re-designed web site, powered by WordPress, my favorite Content Management System (CMS). Visit ncnaapt.org.

If you don’t see a menu on the right side, the new server location probably hasn’t made it to your corner of the internet yet. Come back in an hour or two and refresh the page to see the new revisions.

Force data from elevator ride to top of 555 California St, SF (52 floors)

Graph of change in acceleration in "g's"

Yesterday I took a trip up the longest elevator ride in San Francisco (at 555 California St). I brought along my LabQuest and Force Plate (essentially a recording bathroom scale, see image of LabQuest at bottom of post). As I rode the elevator up and down, the LabQuest was recording my apparent weight (think about how you feel when an elevator first starts moving up or down, your weight seems to change).

I made five round trips, three recording my own weight, then two recording the weight of my Timbuk2 bag (I noticed I was bouncing quite a bit on the ride, and thought my bag might bounce less).

If you have Vernier LoggerPro software (worth the price for their video analysis even if you don’t have any of their sensors), you can download and work with the file. Here it is:

555-california-st-elevator-to-52-floor

I did some data processing in this file; feel free to use it or delete it if you want your students to have to do it all themselves. The graph above was created by dividing the force on the scale by the initial (resting) force, then subtracting one to the the relative change in apparent weight. The maximum and minimum are both about 0.12g, or about 12% of gravity.

A generic image from Vernier's web site.

One discovery I found interesting is that the elevator does not have a continuous acceleration as it gets to its maximum speed, but rather starts with a high acceleration, then decreases its acceleration until it gets to its highest speed. When slowing down, it does the reverse: It starts with a low acceleration, increasing it as it slows until it comes to a stop and stops its acceleration (non-physics geeks: in physics, we don’t use the word “deceleration,” but rather just use positive and negative to tell direction).

I rode up the “express” elevator to the restaurant on the top. The security manager pointed me in that direction once I told him I was just wanting to collect the data for my classroom, not that I wanted to bring a bunch of high school students up with me. The express elevator turned out to be the best option, since I was able to see the maximum acceleration without having to stop at intermediate floors.

Wind Powered Art Vehicle/Kinetic Sculpture

Here’s a video of a great kinetic sculpture that walks along a beach powered by the wind.

Great physics cartoon

Egg drop contest

One of the classic physics projects is an egg drop contest. Students develop an apparatus to hold an egg that will be dropped from the second or third floor (depending on how high the teacher can get easily). This cartoon is a great twist on that, and maybe a reason to use only unfertilized eggs…