Method The equipment was set up as shown in the diagram . A clamp stand was used to support a half metre ruler. The ruler was carefully viewed from all angles to ensure that it was vertical (unlike the one below!).

A spring was fixed onto a screw of the clamp stand equipment and pointer was suspended below it. The mass hanger was hooked under it. A pointer was used to help read the scale on the ruler. It had to be viewed at eye level to avoid parallax error. The experimenter had to be sure the spring system was stationary before a reading was taken. The reading was taken of the position of the pointer when only the mass hanger was suspended from the spring. A 20g mass was then carefully slotted onto the hanger and the reading of the position of the base of the hanger was recorded as before. This was repeated with more 20g masses until 200g had been added to the hanger. The whole experiment was repeated after adjusting the position of the ruler to avoid 'repeat' errors (looking for the value from the previous set of results!)

The results were recorded in a table.

It is important to record all of the readings taken and to show clearly any calculations we do from those readings. We therefore needed four columns.

• We used masses - we therefore needed a column to record the mass we used.
• The weight of each mass added was then calculated and recorded in a separate column. Mass gets pulled by gravity. The force that that produces is called the weight. We can work out the weight exerted by the masses in our results by using the equation:

w = m g

where g = gravitational field strength of Earth = 10 N/kg

As 'g' is given in N/kg we had to change the mass in grammes into a mass in kilograms before we could calculate the weight in newtons.

• The readings from the ruler were recorded in a column and
• the total extension for added weight to the hangerwas calculated from these 'bare readings' on the ruler.

Results table (one was needed for each set of results)

A graph was plotted of the weight added to the hanger against the extension of the spring. The points of both sets of results were plotted on the one graph. This gave us a scatter graph of the results. It showed us not only the trend of the results but also how precise our readings were. A key explaining 'which set was which' was written on the graph.

The title of the graph indicated what the experiment involved - it did not just say what was plotted angainst what - that could be seen from the labelling of the axes!

Anomalous results were plotted, circled and repeated.

A best fit line was plotted. These points indicated a trend of proportionality so a smooth straight line was drawn. It did not matter wherther it went through all of the points. What mattered was that it showed the general pattern that the results indicated.

Conclusion

The amount the spring stretches plotted against the weight added to the hanger gives a straight line that goes through the origin..

This means that the extension is directly proportional to the applied force or load. The proportionality only goes on to a particular point - the elastic limit... after that point the load you put on the spring permanently deforms it and spoils its properties.... it becomes 'stretched out' and then will not return to its original size. Its behaviour is altered. This was first found out by Robert Hooke and we summarise the idea in a Law named after him.

Hooke's Law states that the extension of a spring is directly proportional to the applied load (providing the elastic limit has not been exceeded)

At A level we express the idea in an equation:

We can put this in an equation. The constant 'k' shows it is a proportional relationship.:

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