Science fair: Which shopping bag is strongest per gram? (ages 10–16)

The question

• Which shopping bag is strongest for its weight? Is a thin single-use bag really "weak", or is it just thin?
• Ages 10–16. Time: one 60-minute session for testing plus a write-up. Made and funded by No Plastic Today. Licence: CC BY 4.0.

Why it matters

• Bags are compared per use, not per bag. The UK Environment Agency's life-cycle assessment found a bag for life must be used at least 4 times, a non-woven polypropylene bag 11 times and a cotton bag 131 times to beat a thin single-use bag on climate impact (UK Environment Agency, 2011, executive summary: biodeg.org/wp-content/uploads/2021/04/uk-ea-publishes-lca-of-supermarket-carrier-bags-.pdf). Part of the reason is weight. A single-use HDPE bag weighs only 8.12 g, while a cotton bag weighs 183.11 g, and every gram has to be made. This experiment tests whether that light bag is also weak.

Hypothesis

• Write your own before you start. Example: "The bag for life will carry the most weight before failing, but the single-use bag will carry the most weight per gram of plastic."

Variables

• Independent variable: bag type (single-use HDPE, bag for life LDPE, non-woven polypropylene, cotton, paper).
• Dependent variable: load at failure (kg), and load per gram of bag (kg/g).
• Control variables: how the load is added (same weights, same order, same speed); how the bag is held (same hook or rod through both handles); bag condition (all new); temperature and dryness.

Materials

• Three of each bag type (repeats matter).
• Kitchen scale accurate to 1 g.
• A sturdy rod or hook fixed at about 1.5 m, above a cushioned drop zone (folded cardboard or a crate of rags).
• Weights: bags of sugar or rice (1 kg), tins, or a bucket you add water to (1 litre = 1 kg).
• Data table, camera.

Method

• 1. Weigh each empty bag and record the mass in grams.
• 2. Hang the bag by both handles from the rod so the load hangs freely and cannot rest on anything.
• 3. Add load in 500 g steps. Pause 10 seconds after each step. Record the load when the bag tears, a handle snaps or a seam opens. Photograph the failure point.
• 4. Repeat for three bags of each type. Calculate the mean load at failure.
• 5. Divide the mean failure load (kg) by the bag mass (g) to get kg carried per gram of bag.
• 6. Optional: repeat with the bags after 20 simulated uses (fill with 5 kg, carry across the room, empty; 20 times) to see how strength changes with wear.

Results table (columns)

• Bag type | Mass (g) | Failure load trial 1 (kg) | Trial 2 | Trial 3 | Mean (kg) | Mean load per gram (kg/g) | Where it failed

Analysis questions

• Which bag carried the most? Which carried the most per gram?
• Why might a bag that scores well per gram still be a poor choice for the environment? (Think about how many times each is used.)
• Using the Environment Agency break-even numbers above, how many of your test loads would a cotton bag have to carry before it "pays back"?
• What would you change to make the test fairer?

Safety

• Keep feet out of the drop zone; wear closed shoes. Do not exceed 15 kg. Lift with straight backs. Water buckets: mop spills immediately.

Curriculum links

• England: Key Stage 2 (KS2) Science Year 5 "Properties and changes of materials" (comparative and fair tests); KS3 Science "working scientifically" and design and technology (D&T) (evaluate products); General Certificate of Secondary Education (GCSE) Chemistry (life-cycle assessment).
• Next Generation Science Standards (NGSS): 3-5-ETS1-3 (plan and carry out fair tests); MS-ETS1-4 (develop a model to generate data for iterative testing); HS-ETS1-3.
• Australian Curriculum v9: Science Inquiry (planning and conducting, Years 5–10) (verify codes); Design and Technologies.
• International Baccalaureate (IB) Environmental Systems and Societies (ESS) internal assessment angle: link bag strength and reuse counts to the LCA break-even in a local survey.