Bag Strength

Objective

To determine the strength of a particular type of bag (for example, plastic bags) and exactly how much the strength increases when multiple bags are used.

Difficulty

Procedure: Easy

Concept: Easy

Concept

Take a plastic bag and fill it with stuff until it breaks. Now take two more plastic bags, just like the first one, insert one inside the other and fill that inner bag up with the same amount of stuff that broke the first bag. Will that bag break? Will both break? What do you think will happen?

The idea to test here is whether there is some sort of linear relationship between the amount of weight that bags can hold and the number of bags used. Another way to put it is this: do two bags, one inside the other, hold twice as much as one bag? The other possibility is that there is some sort of exponential growth in the amount of weight that the bags can hold. An example of this would be if two bags could hold three times as much as one bag, and three bags could hold nine times as much as one bag. The final possibility is that there is no change whatsoever. These are the things you need to think about, formulate a hypothesis, and test.

Hypothesis

Which pattern of growth of strength do you think you will see as more bags are added? Think carefully about the concept questions and then formulate your hypothesis.

Materials

Plastic bags are recommended because they are easy to come by in large numbers, but you could try this with paper bags, too, or even garbage bags, as long as you have sufficient weight to test them with.

You will definitely need a lot of weight to test your bags with, preferably many items of the same weight. You can create this simply by filling some Ziploc bags with the same amount of sand. Feel free to use other items or methods as well.

Finally, you will need something to suspend the bag from. An existing nail in the wall (DON'T RUIN YOUR HOUSE) would be a good example of this. You might even be able to hang this from a sturdy coat hanger.

Procedure

  1. The first step is to find the breaking point of one solitary bag. Hang it on your coat hanger or whatever you found and begin filling it. Make sure the scale is underneath, as it will save you a little work later. At some point your bag will give in and break. Note where the bag fails. Was it the handles, or did the bottom fail first? While this is not what you are testing at the moment, it may help with future analysis of your data.
  2. With any luck, when your bag failed the weights inside should have fallen to the scale. If not, just pick them up and put them on the scale (this is why sandbags or Ziploc bags with sand in them are recommended, as they're easily movable and shouldn't make a mess). Record the weight it took to break the bag, then do this same test with at least three more bags. This is to make sure that the bags break at some consistent weight range.
  3. Once you've taken down the breaking weights of at least four single bags (and made sure that they're fairly consistent with each other!) you are ready to try double bags. Insert one bag into another so that the one is basically acting as a liner for the other, then hang both from your hanger over the scale. Again, fill it up until the bag breaks, record the weight, and repeat this trial for double bags at least three times.
  4. After the double bags, try the same procedure at least four times for triple bags and quadruple bags. We encourage you to do as many extra trials as you need to feel completely satisfied with your data. You can even add more bags into the equation. Remember, the more data you take, the easier it will be to draw meaningful conclusions from the data.

Analysis

Take your table of data and plot all the data onto a graph, using the variables number of bags and weight held. Could you possibly draw a straight line through the points? If so, we recommend you check out our Line Fitting Guide for a detailed tutorial.

Conclusions

What does the data tell you about the behavior of bag strength as more bags are used? Was your hypothesis right or wrong?

Extensions

Like we said before, you can keep adding bags and seeing how much more they can hold. You can also try different types of bags and see whether adding bags is as effective for other bag types as it was with the plastic bags.

You could also try this experiment with paper towels. See how much weight you can put on one paper towel before it rips through, and then try it with two, three, and four paper towels. You can tell your parents which paper towel brands are really the toughest!