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Science project idea

Is a 1 kg packet really 1 kg? Sampling and averages with a kitchen scale

No two packets weigh exactly the same, so averages, ranges and standard deviation describe how close the packets are to the printed weight. You weigh sample packets and decide whether the pack is accurate.

Make it yourself7 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Maths.Browse MathsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

No two packets weigh exactly the same, so averages, ranges and standard deviation describe how close the packets are to the printed weight. You weigh sample packets and decide whether the pack is accurate.

The science

Measurements vary because of the packing machine, the scale and moisture. The mean is the sum divided by the number of packets, and the standard deviation measures the spread around the mean. If the mean is close to the label and the spread is small, the packs are consistent, but a scale that is not zeroed will give a biased result.

Materials

Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • 10 to 20 packets of sugar, rice or salt, of the same brand
  • A kitchen scale with 1 g accuracy
  • A notebook or spreadsheet
  • A calculator
  • Graph paper
  • A pencil
  • A bag to carry the packets

Safety

For this project: Do not open the packets and take them back to the shelf or the shop. Ask the shopkeeper’s permission before weighing, and do not accuse any shop of cheating.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Check that the scale reads zero and test it with a known weight, such as five Rs 10 coins.

  2. Weigh each packet and record the weight in grams to the nearest gram.

  3. Calculate the mean by adding all weights and dividing by the number of packets.

  4. Find the range, the largest minus the smallest weight.

  5. Calculate the difference of each value from the mean and square it, add them and divide by the count, and take the square root for the standard deviation.

  6. Draw a dot plot or histogram of the weights.

  7. Compare the mean with the printed weight and write what you can say about the packets.

What you should see

The mean weight is close to the printed weight, with a small spread of a few grams, and the histogram shows a bell-like shape. Differences between packs come from random variation and not from cheating.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is the mean?

The sum of all values divided by their number.

2What does standard deviation show?

How widely the values are spread around the mean.

3Why check the scale first?

An error in the scale would bias every reading.

4Why weigh many packets?

A single packet can mislead because of random variation.

Free ebook: Viva Questions for Science Projects

Take it further

Compare two brands of the same item and find which has the more consistent weight.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

Free ebooks for this project

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