Elastic Material Behaviour Under Tensile Force Investigation

Aim: To investigate the behaviour of an elastic material when a tensile force is applied.

What I know: In the 1660s Robert Hooke investigated how springs and wires stretched when loads were applied. He found out that for many materials, the extension and load were in proportion provided the elastic limit was not exceeded.

Materials can be compressed as well as stretched. If a material is stretched but springs back to its original shape they are known as elastic. However they stop being elastic if bent or stretched too far.

They either break or become permanently deformed.

The springs represent the bonds caused by the forces of attraction and repulsion between the atoms, due to the electric charges of their nuclei and electrons.

The attractive forces between the molecules in a solid provide its characteristic elastic or stretchy properties. When we stretch a solid, we are slightly increasing the spacing of it molecules.

The tension we can feel in a stretched spring is due to all the forces of attraction between the molecule in the spring.

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Possible variables: I could change

> The mass of load

> Material of spring

> Length of spring

Variables I will study: I will study the mass of load.

How I will make my test fair and why: To make sure my investigation is as fair as possible I will do a number of things.

Firstly I will make sure that an equal amount of mass (100g each time) is added each time. Then I will make sure that the readings are taken accurately, by using a splint to measure and mark the exact millimetre on the ruler.

I will also make sure the spring is as close to the ruler as possible and also hanging off the table if the weights exceed past it to, make sure I get an accurate reading. As if it leans on the desk it could effect the extension and the deformation of the spring.

If I have to do the experiment the next day I will make sure that the spring I use is put in a separate place, because the used spring may start to become deformed. If we used a new spring our results could be effected.

Predictions: I predict that every time the mass is increased the extension will increase in proportion. The spring will also start to become slightly deformed and then eventually go beyond the elastic limit, and then become permanently deformed. The constant should be similar for all the masses.

Hypothesis: I predict that the extension will increase each time mass is added on. This is because each time 100 grams more is added on, there is going to be extra force pulling on the spring.

The spring has a point called its elastic limit, which is the point beyond which further extension causes permanent extension.

The extension of a spiral spring is directly proportional to its stretching force.

Once the spring is put beyond its limit it will become deformed. The constant is calculated by

Stretching force / extension.

The extension may double each time so therefore the constant will end up being similar.

Safety: Its important to wear eye goggles to protect our eyes, encase the spring snaps. Also we must take care with the weights, as not to drop them on ourselves.

Apparatus diagram:

Method:

1. I set up the apparatus as above.

2. I measured the length of the spring to the nearest millimetre using a ruler.

3. I marked a line on the ruler for the length of the spring with ought any mass on it.

4. I added a 100g weight onto the spring.

5. I calculated the extension of the spring by subtracting the initial length reading for the unloaded spring form the loaded spring.

6. I then calculated the stretching force which was the mass X N/Kg. The 10N/Kg being the pull of gravity.

7. I then calculated the constant which was done using the formula;

Stretching force / extension

8. I then took the weight off the spring and measured it again using a ruler to see if there was any deformation to the spring.

9. I then repeated the method until the spring was deformed and would not return to its original state.

Results table:

Mass

(g)

100 g

200 g

300 g

400 g

500 g

600 g

700 g

800 g

900 g

1000 g

1100 g

1200 g

1300 g

Extension

(mm)

37 mm

77 mm

116 mm

170 mm

224 mm

268 mm

285 mm

335 mm

382 mm

408 mm

489 mm

511 mm

591 mm

Stretching force

(N/Kg)

1000 N/Kg

2000 N/Kg

3000 N/Kg

4000 N/Kg

5000 N/Kg

6000 N/Kg

7000 N/Kg

8000 N/Kg

9000 N/Kg

10000 N/Kg

11000 N/Kg

12000 N/Kg

13000 N/Kg

Constant

(mm)

27 mm

26 mm

25.9 mm

25.5 mm

22.3 mm

22.4 mm

24.6 mm

23.9 mm

23.6 mm

24.5 mm

22.5 mm

23.5 mm

22 mm

Deformation

(mm)

0

0

0

0

0.5 mm

0.5 mm

1.0 mm

1.0 mm

1.0 mm

1.5 mm

41 mm

59 mm

100 mm

Graph: See graph paper

Conclusion: My graph shows that the gradient is positive as the correlation is going up. This means that the extension has been increased each time more mass has been added on. It is roughly proportional as it is following the trend of the line.

Until 100 grams the spring had only been deformed by 1.5 mm. This part of the graph is straight so therefore it obeys Hookes law.

This is due to the constants staying relatively the same, as the same amount of weight has been added on each time. So therefore the extension and load were in proportion meaning it obeys Hookes law.

There is a proportional relationship between the two variables at the beginning of the graph due to the constant gradient. After 1000 grams is added Hookes law no longer applies because the spring has passed it elastic limit.

The graph shows how the spiral spring stretches as the load hanging on it increases. This is known as a calibration graph.

My prediction was correct. The graph shows that when more weight was added the length of the extension increased. The spring also started to become deformed when the mass gradually increased. The constant was also similar for all the weights. This proves that my prediction was correct.

Evaluation: Although I tried to keep my investigation as fair as possible, I could have done more to make my results more reliable.

When I was measuring the extension of the spring I used a ruler (a metre ruler and also a 30-cm ruler). This meant that measuring to the nearest millimetre was difficult, and I couldn't be sure that I read the correct reading.

I also only started to use a splint to read the extension when I was half way through the investigation. So therefore the ones before will not be as accurate as the one after I started to use the splint.

However the masse that were added on each time (+ 100g) were very reliable as I used a correct weights.

The spring I used was also kept separately from the other springs. This is because using a new spring would effect the results, as the one we started to use would begin to deform.

I think that my method was suitable, because I set up the apparatus and collected all the things I needed before starting the investigation.

I also measured the extension and the deformation in a correct order that suited my experiments.

My measurements were as accurate and reliable as I could make them using the apparatus I had. I made sure that every individual weight was done correctly.

To improve my investigation for next time, in order to get more evidence for my conclusion I will do a number of things.

1. Make sure that a splint is used for all the individual mass

2. Measure the extension and deformation more accurately using a more reliably measuring device.

3. I will repeat the investigation in order to collect more results.

4. Investigate other elastic and plastic materials to see if there is any relationship between their extension and stretching force. I will then be able to comment on the elastic limit of different materials.

5. I could use different length springs.

6. Use smaller weights so I can get a more accurate reading of its elastic limit.