Spanning the Desk

Here is one of the winning paper bridges that the Principles of Physics classes built this month.

Students were given one piece of 8.5 x 11 paper with which to build a bridge that would span 8 inches and hold as many pennies as possible. They could fold their paper any way they wanted, or cut off strips to use as supports.

Before building, classes tried an interactive website at PBS.org to determine which shapes were strongest. Some students used the triangle shape, which had proven victorious in holding the most imaginary elephants on the interactive site. Others chose the arch or circle, and some used the rectangle shape. One crafty group decided to make a bridge shaped like a coin roll. The only problem was filling it with pennies, since the bridge had to be set into position, then filled. It was hard to poke the pennies into a narrow tunnel.

The overall winner had a flat bottom and arched roof, and slowly sagged its way toward the desktop as it filled with pennies, until finally touching bottom and being declared collapsed.

Only one group chose Mrs. Cureton's favorite style, the accordion fold. The first year of building paper/penny bridges, Mrs. Cureton's bridge trounced all competitors by over a hundred pennies. Last year she was soundly beaten by a student using her favorite design.

You will notice that the pictured bridge utilized the triangle shape on the sides for support. The manner in which this bridge failed was the lack of sides to hold in the pennies. One little error in penny placement, and a cascade of pennies leaked over the side, causing the bridge to crease from tension on the paper, and then twist from the torsion force.

With the accordion design, you fold a piece of paper back and forth like a fan. Start putting pennies along the creases, from each end. Once the ends near the supports are full, start working towards the middle, again making sure to apply pennies alternately from both ends. You can see that an accordion design has much more surface area on which to place the pennies than the flat style shown above.

Give it a try, and see if you can beat 247 pennies before collapse.

Speedy Hot Wheels

During April, the Physics classes found a relationship between potential and kinetic energy by driving Hot Wheels down a ramp. Data was used to make a bar graph. The dependent variable, the distance the car traveled past the end of the ramp, was plotted on the y-axis (vertical). The position of the car on the ramp, the independent variable, was plotted on the x-axis (horizontal).

Students discovered that the higher a car was placed on the ramp, the farther the car would roll until it came to a stop. That's because the a car higher on the ramp has more potential energy (energy of position due to gravity) than a car placed lower on the ramp. Once the car is released, the potential energy changes to kinetic energy (energy of motion), which makes the car roll until friction causes it to stop.

There was keen competition for the best rides, but groups selected Hot Wheels from Mrs. Cureton's new car lot at random. Random, meaning that the first picker got one pick, as did others through the first round, then the last picker got to pick three cars, and pickers then went in backwards order, getting two picks apiece. All groups had three cars to test, before choosing the best one for the experiment. Some shrewd groups chose their slowest car, the reason being that they would not have to measure as much distance.

And in case anybody wants to make a note-to-self for future reference, it is NOT acceptable to roll a Hot Wheels down the ramp and up another group's ramp to see if it goes over the top. It will not, because you can't get more energy out of a system than you put into it. Neither is it acceptable to put your ramp up on a desk to see how far your Hot Wheel will fly. That will result in Mrs. Cureton revoking your Hot Wheels license, because it means she will have to operate a used car lot next year.

Special thanks to Mrs. Scandrett for loaning us her ramps.

The Domino Effect

During the month of April, Mrs. Cureton's freshman Physics classes practiced graphing by plotting the speed of falling dominoes. Students placed the dominoes different distances apart (close, medium, far), measured the length of the line, and then toppled them while timing the result. A bar graph was used to interpret the data. Speed, the dependent variable in this activity, was displayed on the y-axis (vertical). The independent variable, the distance apart of the dominoes, was displayed on the x-axis (horizontal).

Some groups discovered that closely-spaced dominoes fell faster, and some groups found that farther-spaced dominoes fell faster. In a perfect experiment, under controlled conditions, there is an optimal distance of 2 cm at which a straight line of dominoes falls the fastest. More closely spaced dominoes fall slower because it takes them longer to fall over, and more distantly spaced dominoes fall slower because it takes them longer to hit the next domino.

If you want to study this phenomenon in detail, check this out. Or for a little bit simpler version, look here. In our class, we did not measure the spacing between dominoes. Mrs. Cureton has found that procedure to be too time-consuming for the classroom.

















Students also discovered which members of the group had a steady hand for setting up dominoes. Some found this out the hard way! Likewise, they found out who had a quick thumb on the stopwatch...or better yet, who actually remembered to hit the button when the dominoes were toppled.