Plastic wrap it has been protecting leftovers for decades. Using household items, you can conduct an experiment to test the strength of plastic wrap.

CONCEPTS

STRESS is the ratio of the applied load to the cross-sectional area. Because the width of the plastic wrap is much larger than its thickness, we can simplify the equation to:

stress = load / width

STRAIN is the ratio in the change in length due to an applied to the original length.

strain = (length with applied load – original length) / original length

HOOKE’S LAW describes the linear relationship between stress and strain for an elastic material.

stress = constant * strain

The YIELD STRENGTH is stress at which a material no longer behaves elastically meaning it does not follow HOOKE’S LAW.

The maximum load that a material can withstand before it breaks is called the ULTIMATE STRENGTH.

MATERIALS

*Plastic wrap

*Scissors

*Ruler

*Popsicle/Craft Sticks

*Binder clips

*Washers or pennies to use as weights

*Paper cup

*String

*Tape

*Paper/Graph paper

*Pens/pencils

*Kitchen scale

SET UP

Tape several craft sticks together. Secure to the top of a table with a short segment sticking out to use as a hook. This can be done with strong tape and placement of a heavy object such as a phone book for extra security. Tape together enough pieces of paper/graph paper to span the distance between the top of the table and the floor. Secure the paper to the table and floor with tape.

Make a basket for holding the weights by cutting small slits on either side of the paper cup and sliding a craft stick through. Weight the basket and craft stick on a kitchen scale.

If you are using washers, estimate the average using a kitchen scale. If you are using pennies, the weight of a penny minted before 1982 is 3.1 g and 2.5 g thereafter.

Cut three strips of plastic wrap parallel to the cutting edge of the container. Each strip should be 2.5 cm (1″) wide and 30.5 cm (12″) long. Wrap 5 cm (2″) of the ends of each strip around a craft stick and secure with a binder clip leaving 20 cm (8″) between the craft sticks. The metal loops on the binder clips should be flipped up and away from the opening of the clip.

EXPERIMENT

Hang a sample of the plastic wrap by sliding the metal loops over the hook attached to the table.

Mark the length of the plastic wrap sample on the paper hanging from the table. To do this, mark the inside edges of plastic wrap sample at the edges of the craft sticks on the top and bottom.

Hang the basket from the bottom of the plastic wrap sample by sliding the craft stick through the loops of the binder clip.

If the plastic wrap changes lengths, mark the new position of the bottom craft stick.

Add weights to the basket marking the new length each time. You make need to add a few at a time to see any change in length. Be sure to keep track of the number of weights in the basket each time you mark the length.

Continue to add weights until the strip breaks. Record the weight in the basket when the strip breaks.

ANALYZE THE DATA

Question 1: Does plastic wrap follow Hooke’s Law?

Using the equations above, calculate the stress and strain for each data point collected. Be sure to account for the weight of the basket.

Plot these on a graph with strain as the x-axis and stress as the y-axis. This can be done on graph paper or using a computer program like Excel. Be sure to use different colors and/or symbols of each strip of plastic wrap tested.

If plastic wrap follows Hooke’s Law, the beginning of the graph for each strip should be a straight line. Calculate the constant in Hooke’s Law for each strip of plastic wrap.

Question 2: What is the yield strength of plastic wrap?

Looking at the graphs from Question 1, determine if there is a point where the graph is no longer a straight line or the line changes slope. This is the yield strength. Record the yield strength for each strip tested.

Question 3: What is the ultimate strength of plastic wrap?

Using the equation above, calculate the stress when the plastic wrap broke. This is the yield strength. Record the yield strength for each strip tested.

MORE EXPERIMENTS

Not all plastic wraps are the same. Try this experiment with different brands and compare the results. What might account for the differences you observe?

Some materials are stronger in different directions. Try measuring the strength of plastic wrap in different directions, for example perpendicular or at an angle to the cutting edge of the role.