Material Strength Experiment for Kids: Test Plastic Bags

The thin crinkly supermarket bag that everyone complains about will usually out-pull the soft stretchy bread bag that feels twice as tough. That is the first surprise in this material strength experiment kids can run with a broom handle and a water jug. The second one is better. Cut a 2 mm nick in the edge of a strip that just held a kilogram and it will fail at half that, or less. Engineers have a name for what you have just demonstrated, and it is the reason aircraft inspectors care about tiny scratches.

Key Takeaways

• You’ll cut identical 2 cm strips from five different bags, hang them up, and add water until each one breaks.
• It teaches tensile strength and the difference between force (how many kilograms) and stress (force divided by cross-section), which is the number engineers actually quote.
• Best for ages 8–14. Younger kids can do the pulling; the stress calculation needs about fifth-grade division.
• The usual mistake: cutting strips of different widths, which makes the comparison meaningless.
• Plastic film is not the same strength in both directions, and finding that out is the best part.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Thin supermarket carrier bag (HDPE, crinkly)	1	Kitchen drawer	Any crinkly plastic bag
Soft stretchy bag (LDPE, bread or produce)	1	Kitchen	A bin liner
Zip-seal food bag	1	Kitchen	A freezer bag
Brown paper grocery bag	1	Kitchen	A large envelope
Woven reusable shopping bag	1	Car boot	A sheet of landscape fabric
Broom handle or wooden dowel	1, about 1 m	Cupboard	A sturdy curtain rod
Two chairs of the same height	2	Kitchen	A door frame and a hook
Bucket or sturdy carrier bag to hold weight	1	Kitchen	A reusable tote
Water and a measuring jug	5 L and a 250 mL jug	Tap	Bags of rice or sugar in known weights
S-hook or carabiner	1	Hardware store, pocket change	A bent coat hanger
Ruler, scissors, marker	1 each	Kitchen drawer	Nothing else needed
Kitchen scale	1	Kitchen	Skip it and use measured water: 250 mL is 250 g
Cushion or folded towel	1	Anywhere	Place it under the bucket

Time: prep 20 min, experiment 30 min, cleanup 5 min Cost: everything is probably in your kitchen Mess level: low indoors, lower outdoors. Water will spill when things snap

Do the Experiment: A Material Strength Experiment Kids Can Measure

Step 1 — Cut five strips exactly 2 cm wide and 20 cm long

Width is the whole experiment. Rule a line, cut along it, measure the strip, and recut if it is off by more than a millimetre. Cut all five strips in the same direction relative to the bag: from the opening down towards the base. Write the direction on each strip with the marker.

Step 2 — Tie each strip into a loop

Overlap the ends by 3 cm and tie a simple overhand knot, or fold the overlap and staple it twice. A knot concentrates the load, so every strip must be knotted the same way. If you have a strip that keeps breaking at the knot, that is a knot result, not a material result.

Step 3 — Rig the test

Lay the broom handle across the backs of two chairs. Hang a loop over the handle, hook the S-hook through the bottom of the loop, and hang the empty bucket from the hook. Put a cushion underneath. Nobody sits, kneels or puts feet under the bucket.

Step 4 — Add water 250 mL at a time and record

Pour 250 mL into the bucket, wait five seconds, pour another. Count out loud. When the strip snaps, the last recorded total is the breaking load. Note the weight of the empty bucket too and add it in.

Step 5 — Repeat for all five materials, three strips each

Three strips per material. Film is not uniform, and one strip tells you almost nothing. Average the three and write down the spread as well as the average.

Step 6 — The nick test

Take a fresh strip of whichever bag performed best. Cut a 2 mm nick into one edge, halfway along. Test it the same way.

Step 7 — The direction test

Cut three more strips from the same bag, this time across the bag rather than along it. Same width, same knot, same bucket.

What you should see: A single 2 cm strip of a thin supermarket carrier bag usually fails somewhere between 0.8 and 1.5 kg, and it stretches very little before it goes. A soft bread bag stretches a long way, sometimes narrowing into a thin ribbon, and typically breaks at a lower load. Paper doesn’t stretch at all and snaps cleanly, often at a few kilograms. The nicked strip fails at a fraction of the intact one, frequently under half, and it tears from the nick every time. The across-the-bag strips often behave completely differently from the along-the-bag strips, sometimes stretching three times as far.

What’s Actually Going On

Tensile strength is the stress a material can take before it breaks, where stress means force divided by the cross-sectional area carrying it. The units are newtons per square millimetre, also called megapascals. That definition is why your bucket reading alone cannot rank materials: a thicker strip holds more kilograms without being a stronger material.

This is exactly what the standard test method ASTM D882, “Standard Test Method for Tensile Properties of Thin Plastic Sheeting,” is built around. It specifies the strip width, the distance between the grips, the rate at which the load is applied and the requirement to report results as stress. Your broom handle is a rough version of that machine, and the three things the standard fixes are the three things most kitchen-table versions get wrong.

To get from kilograms to megapascals you need the film thickness, and you do not need a micrometer. Cut a 10 cm by 10 cm square, weigh it on a kitchen scale, and divide: thickness equals mass divided by area times density. Polyethylene density is about 0.95 g/cm³ for the high-density kind and 0.92 for the low-density kind. A 10 cm square weighing 0.14 g works out to roughly 15 micrometres, which is a typical carrier bag.

Now the direction result, which is the real physics. Carrier bags are made by blown-film extrusion: molten polyethylene is forced through a ring die, inflated into a bubble and pulled upwards. Both the pulling and the inflating stretch the polymer chains, so the molecules end up partly lined up with the machine direction. Chambon and Srinivas measured this orientation in cast and blown polyethylene films in 1999, and Guleria and colleagues showed in 2024 how much the draw ratio changes the resulting mechanical properties. Aligned chains carry load well along their length and poorly across it. That is why a bag tears neatly in one direction and stubbornly resists in the other, and why your two sets of strips disagree.

The nick is a separate idea and a bigger one. A notch does not merely remove material; it concentrates stress at its tip, so the local stress there is far higher than the average across the strip. Alan Griffith worked this out for brittle solids in 1921, showing that fracture starts from flaws rather than from material failing uniformly. It is why packaging has deliberate tear notches, why a scratched windscreen cracks from the scratch, and why engineers inspect for cracks measured in millimetres. “It had a tiny flaw” is not a weak excuse. It is the mechanism.

How to Explain It to Your Kid

Ages 5–8: thin strips, fair races

Let them cut and pour. Keep the language physical: we are pulling until it breaks, and we want every strip to get the same fair chance, so they all have to be the same width. Have them feel the stretchy bag going long and thin before it snaps, and the paper going nowhere before it tears. Two different ways to be strong.

Ages 9–12: strength is force divided by area

Introduce the division. Two kilograms held by a strip twice as thick is not twice as strong; it is the same strength. Have them measure the 10 cm square, weigh it and compute thickness, then convert their bucket readings into stress in megapascals. Comparing a plastic bag with a paper bag only becomes meaningful at that point, and they will notice the two land closer together than the kilograms suggested.

Ages 13+: orientation, flaws and energy

Give them the whole picture: molecular orientation from blown-film extrusion, stress concentration at a notch, and the distinction between strength and toughness. A paper strip and a plastic strip can break at a similar stress while the plastic absorbs far more energy on the way, because energy is force times distance and the plastic stretched. Have them estimate that energy from their own numbers. Then ask which property matters for a shopping bag, and which for a climbing rope.

The question to ask: “If I gave you a bag strip that already has a small tear in it, and you had to carry something heavy, would you rather cut the torn part off and use a narrower strip, or keep the full width with the tear in it?”

Why Teaching This Early Pays Off

Children, and plenty of adults, use “strong” as a single word covering stiffness, strength, toughness and hardness. Those are four different measurable properties, and conflating them produces bad reasoning for years: that stretchy means weak, that thick means strong, that a small crack is cosmetic. Separating them early gives a kid a vocabulary that pays off in middle-school physical science, in any design-and-technology class, and in the first engineering course they ever take.

The flaw idea has reach well beyond materials. Systems often fail at their worst point, not their average one, and a child who has watched 2 millimetres of nick halve a strip’s strength has a physical memory for that.

Two pieces of learning research are worth being honest about here. Alfieri and colleagues’ 2011 meta-analysis found guided discovery helps while unassisted discovery generally does not, and Kirschner, Sweller and Clark argued in 2006 that leaving novices to work things out alone is an unreliable way to teach. So do not just hand over the scissors and hope. Ask for a prediction, give the explanation at the right moment, and have them say it back. The National Research Council’s K-12 framework names the relevant practices as planning investigations and analysing data. One afternoon of pulling bags apart will not make anyone an engineer. It can permanently change what “strong” means to them.

Before You Start: What Parents Should Know

Best age	8–14. Under 8 the arithmetic is out of reach, though the pulling and the nick test still land
Supervision	Close. Scissors, and a bucket of water that will drop without warning
Real hazards	A falling bucket on toes; a snapping strip can whip back towards faces, so stand to one side; plastic bags are a suffocation risk for babies and toddlers, so clear them away afterwards
Don’t	Don’t use glass jars or anything breakable as the weight, and don’t hang the load above a foot, a pet or a laptop

When It Doesn’t Work

• Every strip breaks at the knot. The knot is concentrating stress, which is the nick effect in disguise. Fold the overlap over a short pencil and tape it instead, so the load spreads.
• The results jump around wildly. Normal for film. That is why you test three strips. If the spread is more than about 30%, check that your widths really are identical.
• The bucket hits the floor before anything breaks. Raise the rig: put the broom handle across a door frame, or use a shorter loop.
• The stretchy bag never actually snaps, it just keeps stretching. Record the load at which it visibly necks down to a thin ribbon and call that your yield point. That is a real measurement, and it is a different property from breaking strength.
• The paper strip tears from the edge immediately. Scissor cuts leave tiny notches in paper. Cut cleanly in one pass, or tear it along a folded crease instead.

Take It Further

1. Change one variable: width. Test 1 cm, 2 cm and 4 cm strips of the same bag. Plot breaking load against width. If it is roughly a straight line through the origin, you have just demonstrated that stress, not force, is the material property.
2. Change the temperature. Put three strips in the freezer for an hour and three in a warm spot, then test both. Polyethylene gets stiffer and more brittle cold. The change is big enough to see.
3. Pre-stretch a strip. Pull one strip gently until it necks down, then test it. The stretched region is often stronger per unit area than the original because you have aligned the chains yourself, which is the same trick the factory uses.

Pulling things apart is tension. For the other half of the story, where shape decides whether a material survives being squashed, see spaghetti towers and why they fail and how much a paper bridge can hold. The same polymer chains that make a bag stretch are the reason slime behaves as it does, covered in our polymer chemistry with slime piece.

What to Watch For Over the Next 3 Months

• Week 4: They notice the tear notch on a packet of crisps, or complain that a bag “started from the little hole”.
• Month 2: The phrase “it depends how thick it is” shows up when someone claims one material is stronger than another.
• Month 3: They apply the flaw idea somewhere unrelated: a cracked phone screen, a frayed shoelace, a chipped plate. That transfer is the point, and it is worth following with a question rather than a correction.

Frequently Asked Questions

Do I need a kitchen scale for this?

No. Measured water is a scale: 250 mL of water weighs 250 g to within a fraction of a percent. You only need the scale for the thickness trick, where you weigh a 10 cm square to work out how thin the film is.

Why did the flimsy bag beat the thick one?

Probably because you measured force, not strength. Divide each breaking load by the strip’s cross-sectional area and the ranking often changes. It can also be the material: high-density polyethylene is genuinely stronger per unit area than the low-density kind used for soft bags.

Is it safe to let an eight-year-old do the pulling?

Yes, with the bucket over a cushion and nobody’s feet underneath. The real risk is not the load, it is the recoil when a strip snaps, so have everyone stand to the side rather than directly over it.

What is the difference between strong and tough?

Strong means it takes a high stress to break. Tough means it absorbs a lot of energy before breaking. A biscuit is fairly strong and not at all tough. A bread bag is weak and quite tough. The bags in this test will show you both.

Can we test fabric, string or tape as well?

Absolutely, and the comparison is good. Keep the width constant and expect string to beat everything on force while being hard to compare on stress, because working out the cross-sectional area of a twisted cord is genuinely awkward. That difficulty is itself a useful discovery.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. ASTM International. ASTM D882-18, “Standard Test Method for Tensile Properties of Thin Plastic Sheeting.” https://www.astm.org/d0882-18.html
2. Chambon, F., & Srinivas, S. (1999). “Orientation Analysis of Cast and Blown Polyethylene Films.” Journal of Plastic Film & Sheeting, 15(4), 297–307. https://doi.org/10.1177/875608799901500405
3. Guleria, A., et al. (2024). “Impact of molecular architecture and draw ratio on enhancement of targeted mechanical properties of blown films.” Journal of Plastic Film & Sheeting, 41(1), 35–67. https://doi.org/10.1177/87560879241293392
4. Griffith, A. A. (1921). “The phenomena of rupture and flow in solids.” Philosophical Transactions of the Royal Society of London, Series A, 221, 163–198. https://doi.org/10.1098/rsta.1921.0006
5. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
6. Kirschner, P. A., Sweller, J., & Clark, R. E. (2006). “Why Minimal Guidance During Instruction Does Not Work.” Educational Psychologist, 41(2), 75–86. https://doi.org/10.1207/s15326985ep4102_1
7. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165