Hooke's Law: Explaining Why Weight Changes Cause Different Spring Movement

[ConquestAce]

How come a spring with applied weight of 1000 g goes down 9.8 cm but the same spring with applied weight of 500g goes only 3.7 cm down?

This is from my own lab that I preformed, and I need help in explaining why this occurs. I searched online but didn't get any clear answers, my only assumption at the moment is that k constant is the reason why this occurs, but I am not too sure.

The K values for the same spring is different when I calculate it, and I am confused on why that happens.

 

[paisiello2]

Obviously the spring doesn't follow Hooke's Law completely. More than likely it is following Hooke's Law for small loads but it starts to yield at higher loads.

 

[jtbell]

An interesting experimental followup would be to take the same spring and put 100g, 200g, 200g, ... 900g on it, measure the amount of extension for each, and plot a graph. Does it start out as more or less a straight line, and then start to curve with larger weights, or does it curve all the way?