Investigating the Strength of a Supermarkets Plastic Bag

Aim

I have been asked to investigate the strength of a supermarkets plastic bag. I am going to test and analyse the plastic bag by investigating how applying a gradual increasing force effects the extension of the material.

Apparatus

* 30cm by 1cm strip of plastic bag,

* Stand,

* Clamp,

* Boss,

* Weights (N)- 100g

* 2 wood blocks,

* Ruler

Method

Before we can begin the experiment my group and I will have to set up the apparatus as shown on the diagram but with excluding the weights.

We will set up the apparatus by cutting a 1 by 30cm strip of plastic from a plastic bag. We will proceed by placing the strip in between the two wooden blocks and then into the clamp. We will then begin the experiment by applying the first weight of 1 Newton. After applying the first weight we will then measure the length of the plastic strip record the length and calculate the extension. We will continue the experiment by adding another 1 Newton and recording the result and extension again.

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We will continue to keep adding 1 Newton and recording the results until the strip of plastic snaps or we have recorded a total of 10 results.

We will redo the experiment twice again so that the results are more accurate and we can determine an average. To make sure that we keep it a fair test we will make sure that we use the same length of plastic strip each time we carry out the experiment. When we increase the force we will always increase by 1 Newton to keep it a fair test.

We will be measuring the length and extension of the plastic strip in centimetres (cm). We are doing to this to measure it.

Preliminary Work

Before completing the plan we tested the experiment as explained in the method. We found that as we increased the force in 1 Newton that the plastic strip actually snapped under an average of 3 Newton's which didn't give us enough results for the experiment. We tested this 3 times and found that this was happening for each time we carried out the experiment. To compensate for this we have decided to increase the force by only half a Newton so that more results can be recorded. Whilst carrying out the experiment we did record a set of results as follows-

Experiment 1

Force(N)

Length(cm)

Extension(cm)

0

28

0

1

28.4

0.4

2

28.8

0.8

3

29.3

1.3

Experiment 2

Force(N)

Length(cm)

Extension(cm)

0

28

0

1

28.2

0.2

2

28.6

0.6

3

29

1

Experiment 3

Force(N)

Length(cm)

Extension(cm)

0

28

0

1

28.5

0.5

2

28.7

0.7

3

28.9

0.9

4

29.2

1.2

From these preliminary results I can see that the plastic extends to no more than 1.3cm of its original length before snapping. These results will help me with my prediction.

Prediction

I already know that the plastic bag is made up of polymers. These are chemical bonds which make up the properties that form the plastic bag. The polymers in which plastic bags are made are more commonly known as polyethylene. Polyethylene is a straight chain polymer. It consists of chains made up of only carbon and hydrogen. The bond that form the chain are covalent bonds and are held together by relatively weak electrostatic forces, because the bond carbon and hydrogen that form the polyethylene are covalent it means that they share electrons. The displayed formula for polyethylene is

The chains can go on and on,

The longer the chains the stronger the plastic.

Polyethylene is very cheap to produce. It is strong and easily moulded.

There are forces acting between the particles or molecules of the polyethylene which prevent the change in shape of the material, which when forces are applied can effect. Under the action of external forces the polymer bonds stretch and begin to straighten. At this point the plastic is actin as an elastic. As more force is applied will continue straightening until they reach a point of equal force, at this point the plastic is in a state of strain and can no longer straighten out. This is where the plastic begins to act as a plastic. If more force is applied the bonds will snap and the plastic strip will also snap.

After taking into account of the information I already know and the results from my preliminary work, I predict that as the force is applied to the plastic strip it will stretch as an elastic until the force becomes so great it reaches a point at which it can no longer stretch as the bonds have been straightened and weakened, at this point I believe that the plastic strip will snap.

Results

Whilst carrying out my experiment I recorded the following results,

Experiment 1

Force(N)

Length(cm)

Extension(cm)

0

28

0

1/2

28

0

1

28.2

0.2

1-1/2

28.2

0.2

2

28.4

0.4

2-1/2

28.4

0.4

3

28.6

0.6

3-1/2

28.7

0.7

4

29.0

1

Experiment 2

Weight(N)

Length(cm)

Extension(cm)

0

28

0

1/2

28

0

1

28.2

0.2

1-1/2

28.3

0.3

2

28.3

0.3

2-1/2

28.5

0.5

3

28.6

0.7

3-1/2

28.8

0.9

4

29.0

1.1

Experiment 3

Weight(N)

Length(cm)

Extension(cm)

0

28

0

1/2

28.1

0.1

1

28.1

0.1

1-1/2

28.2

0.2

2

28.3

0.3

2-1/2

28.5

0.5

3

28.7

0.7

3-1/2

28.9

0.9

4

29.1

1.1

I am now going to calculate the average for the extensions of each experiment,

Average results

Weight(N)

Average extension(cm)

0

0

1/2

0.03

1

0.1

1-1/2

0.2

2

0.3

2-1/2

0.4

3

0.6

3-1/2

0.8

4

1.0

I am now going to plot a line graph for each set of results and my average results so that they can be easily viewed and analysed.

I will use the line graphs to look for patterns and trends.

I can also use the line graphs to determine whether I experienced any anomalous or irregular results.