9th – 12th Grade
9th – 12th Grade
The Balanced Equation
Don’t Be a Beach Bum
Wet or Dry Magic Fork Experiment
Spinning Yo-Yo Experiment
Island Survival
Identification of Common Scented and Unscented Products
Physical Properties of Food Wraps
Procedures
Optional Exercise

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Procedures

At the end of this section there are two documents you may download and print –
  1. Procedures
  2. Data and Observation Charts

Tensile Strength

  1. spring scale
    Figure A
    Assemble two ring stands with a crossbar attached horizontally using clamps. Weigh down or clamp the base of the ring stands for stability. See Figure A.
  2. Cut 2-inch x 8-inch strips of each type of food wrap and use duct tape to suspend each strip from the horizontal crossbar.
  3. Attach a second piece of duct tape at the base of each sample and pierce a small hole in the tape. A hooked spring scale will be suspended to measure force.
  4. To measure tensile strength, attach a spring scale to the sample. Pull down at a constant rate until the sample wrap breaks. Record the force (N) at the moment of break. This will give a relative value for tensile strength. The final tensile strength value is usually obtained by dividing the force by the cross-sectional area, but in this case the cross-sectional areas of the samples should be fairly uniform.
  5. Collect all the tensile strength data from the class and calculate the average value ± the deviation.
  6. Repeat the procedure for each type of food wrap.

Elongation

  1. spring scale with ruler
    Figure B
    Use the assembled ring stands with a crossbar from the tensile strength procedure above.
  2. Cut 2-inch x 8-inch strips of each type of food wrap and use duct tape to suspend each strip from the horizontal crossbar.
  3. Attach a second piece of duct tape at the base of each sample and pierce a small hole in the tape. A hooked spring scale is suspended to measure force.
  4. To measure elongation, attach a spring scale to a sample and vertically pull down at a constant rate. Record the force (N) at various points of the stretch (cm) until the sample wrap tears. See Figure B.
  5. Graph force (N) versus stretch (cm). Determine the slope of the graph to get a relative measure for the elongation of each type of food wrap. Record.
  6. Repeat the procedure for each type of food wrap. Record.

Flexural Strength

  1. weight
    Figure C
    Cut a sample of each wrap large enough to be secured over the mouth of an open-ended coffee can.
  2. Mount the sample wrap over the mouth of the can and adhere it to the can with duct tape. Make sure the sample is pulled taut. See Figure C.
  3. Add weight (vertical stress) to the center of the sample until the wrap breaks. Record this force (weight) in newtons. It will be used to calculate the flexural strength.
  4. Use πr2 to compute the area of a circle in cm2 (area of can). Divide the force (N) by the area (cm2) to get the flexural strength.
  5. Repeat the procedure for all sample wraps.

Impact Resistance

  1. hammer action
    Figure D
    Use steps 1 and 2 from the flexural strength procedure to prepare samples for impact resistance. You may need several samples of each wrap.
  2. Construct an impact resistance assembly using a hammer, an eyelet, and the crossbar assembly from the tensile strength procedure. Screw the eyelet into the handle of the hammer such that the crossbar will allow the hammer to swing freely, like a pendulum. See Figure D.
  3. Determine the mass of the hammer (in kilograms).
  4. Adjust the height of the crossbar so that the hammer’s head strikes the center of the mounted sample on the face of the can, at the base of the hammer’s swing.
  5. Pull back the hammer to a set height above the base of its swing. Record height (in meters).
  6. While another lab partner holds the sample still, release the hammer, using caution to avoid injury. Change the starting height until the sample wrap breaks. (A fresh sample should be used for each change in starting height. Otherwise, the sample may be stressed enough by the first hit that a second hit of equal value will break it.) The height will be used to calculate the energy (potential) exerted per sample area required to break the sample. This is a relative measure of the impact resistance of the food wrap.
  7. Calculate the impact resistance (joules per square centimeter) using the potential energy formula (mgΔh) divided by area of the hammer head (πr2).
  8. Repeat the procedure for all sample wraps.

Download the Procedures (125KB PDF)

Download the Data and Observations Chart (60KB PDF)

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