Hooke’s Law

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.

So you could do a simple investigation into Hooke’s Law. Take a piece of very thin wire and hang it on a hook in the ceiling and add a small weight to the bottom of the wire: just heavy enough to pull the wire straight.

  1. Measure the length of the wire,
  2. Add a larger weight to the wire and re-measure the length,
  3. Repeat this with heavier weights,
  4. Plot a graph showing the length of the wire for each weight.

If you have understood Hooke’s Law, you shoulod be able to predict what will happen before you start the investigation. If Hooke’s Law is true, your graph should show a straight line. Of course, it would be a good idea to repeat the experiment a couple of times and get an average length for each weight.


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