| Hooke's Law states that
in an elastic material strain is proportional to stress.
The point at which a material ceases to obey Hooke's Law is known as its elastic limit.
The first part is very easy. It means that the bigger the weight
(stress) you hang on the string the more it will stretch (strain).
The second part is also easy. Whilst the elastic limit is not exceeded, the string will go back to its original length when you take the weights off it, but if you add too much weight, the string will stretch without going back to its original length when you take the weights off it. If you leave a very large weight hanging on the string, it will gradually get longer and longer until it breaks.
In this state the wire is behaving as if it were a fluid instead of a
solid.
For this investigation you also need to know about Young's modulus. This is the coefficient of elasticity of stretching. It is the ratio of the
stress or stretching force per unit cross sectional area to the strain or amount of stretching per unit of length.
You can look Young's modulus up for any elastic material and see if the result you get in your experiments fit. So make sure that you measure the length of
your wire and its diameter. In fact if you know Young's modulus for the
material you are using you will be able to predict exactly how much your
wire will stretch when you hang masses on it.
|
