This week we'll finish the Newton's Second Law lab and initiate our bridge building contest.
Second Law lab -- data collection finishes Monday. If you are having trouble getting all the trials I've suggested try these six as a bare minimum: with the empty cart try with 10 grams (2 clips), 30 grams (2 clips + 20) and 50 grams (2 clips + 2 -20s). Then with the .5 kg mass on the cart try the same three masses on the clips.
Product for submission: 2 graphs, one for the empty cart and one for the loaded cart. Graph paper here. Each graph should have two lines. The axes are accelerating force on the y-axis and rate of change of momentum for the x- axis. Draw two lines, a dotted line to show the theoretical (no friction) prediction and another to show the real data. Use a ruler to put in both lines. The theoretical line should have a slope of 1 and a y-intercept at the origin. The experimental line should be labeled with its slope and its y-intercept determined (it may be negative so leave room on your graph to accommodate that possibility. Due Friday. Last day of the quarter so no use getting this one in late.
Bridge Contest
Every year the Illinois Institute of Technology hosts a contest to see who can build the most efficient model bridge from a particular set of materials. Tinley Park will send four representatives to the contest in February. We will have two rounds of competition to choose our representatives. It is my hope that our four representatives will receive some training from a professional structural engineer prior to building their contest bridge.
Our first round will be a class assignment. To save the costs associated with using basswood, we'll use pasta for our first bridge. This will be first of all a class assignment and second of all a talent search for TPHS contest reps.
Assignment. Using fettuccini noodles build a bridge to the following specifications:
1. Use only fettuccine noodles and glue. I'll provide the first 1 oz of noodles and the glue that we'll use in class.
2. The mass of your finished bridge may not exceed 35 grams.
3. Your bridge must span 15.0 cm (so make it at least a little longer so the ends can rest on the provided supports).
4. Your bridge must be at least 3.0 cm wide.
5. Your bridge must accommodate a block 2.54 cm wide x 2.54 cm high x 10.16 cm long (1 inch x 1 inch x 4 inches).
6. It must be possible to place the center of the block at the center of your bridge's support plane (that is centered both lengthwise and across the width of your bridge).
7. Your bridge must accommodate a 1/2" in diameter rod vertically through the center point.
8. Scoring: you get 5 pts for satisfying steps 1 - 7. Efficiency is determined by dividing mass supported by mass of bridge. You get 1 extra point for each 40X efficiency (that is: efficiency = 40 then +1, 80 +2, 120 +3 160 +4 and 200 +5). No additional points towards the standard.
9. We will work Thursday and Friday in class to build a first bridge. If you would like to build a second bridge (or a third or a fourth) you have until November 1st to submit. This must be made of materials (fettuccini noodles) that you obtain yourself (and your own glue of choice).
10. The ten highest scoring (as of November 1) will be invited to participate in the second round and will build bridges of basswood.
Lots of good stuff on bridge and truss design on the web.
http://www.garrettsbridges.com/design/truss design/
http://www.garrettsbridges.com/files/5steps.pdf
http://www.pghbridges.com/basics.htm
http://www.en.wikipedia.org/wiki/Lattice_truss_bridge
http://www.britannica.com/EBchecked/topic/79272/bridge/72070/Truss-bridges
http://www.jhu.edu/~virtlab/bridge/bridge.htm
http://science.howstuffworks.com/engineering/civil/bridge1.htm
http://www.phlf.org/downloads/education/Edu_WabashBridgeDesign.pdf
http://en.wikipedia.org/wiki/Truss_bridge
http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.lp_tension/
http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.bbskyscraper/
Assignment
Read sections 18.3 and 18.4 of your textbook.
Answer review questions 9 - 15 and think and explain questions 5 and 7 at the end of chapter 18.
Explain the difference between tension and compression.
A1. Look up stress and strain on the Internet (maybe here: http://en.wikipedia.org/wiki/Stress–strain_curve) and explain the difference between them.
A2. Look up the term "chord" within the context of a bridge "truss" and explain how the terms are related. (Maybe here to start: http://en.wikipedia.org/wiki/Truss).
A3. Using the data below, calculate the percentage difference between the theoretical force provided by the hanging mass and the experimental force verified by the rate of change of momentum of the accelerating cart. Speculate on the origin of the difference.
hanging mass = 25 grams
mass riding on track = .500 kg
total mass =
during acceleration: t1 = 0.22 s, v1 = 0.18 m/s, t2 = 1.45 s v2 = 0.73 m/s