Supermarkets need to ensure that they know what the maximum weight their bags can hold. This is important so they don’t waste money making their bags too strong (especially if they are free). But they also need to provide bags which can hold a reasonable amount of shopping. Most of the force is applied in the handles; having handles too thin could result in them snapping, too thick is wasteful.
The experiment below is one which you could try in the lab and is similar to the method used by real scientists to test the strength of plastic bags.
Hypothesis
Different widths of plastic bags will affect how much mass they can carry.
Independent variable – The width of the plastic bag.
This can be measured with a ruler and cut with scissors. The resolution of the ruler used should be 1mm rather than 1cm.
The range could be anything between 5mm and 100mm. For example, 10mm to 70mm or 5mm to 50mm.
The interval should be every 5mm or every 10 mm.
Dependent variable – The mass it can carry.
You will use slotted masses to do this. The resolution could be 10g or 100g.
Control variables
- The type of bag – different bags are made of different polymers. The different polymers will have different strengths.
- The length of strip – It is likely that changing the length will affect the distribution of force through the strip.
- The orientation of the bag – the polymers are long chain molecules joined by cross bridges. Changing the orientation (from up/down to across) will affect the strength.
Equipment
- Plastic bag
- Ruler
- Scissors
- 100g masses
- Retort stand, boss & clamps
- Pair of material clamps
Method
1/ Cut the plastic bag into a 300mm by 200mm piece.
2/ Measure and cut a 70mm by 200mm strip.
3/ Secure the strip into the material clamps.
4/ Place the hanger onto the bottom hook.
5/ Add 100g masses at a time and measure the maximum mass that the strip can hold.
6/ Repeat with 60mm, 50mm, 40mm, 30mm, 20mm and 10mm strips