# Why is the stiffness constant k not a material property - A level physics materials

I ticked Young's modulus and stiffness constant. Answer was breaking stress and youngs's modulus. Can someone explain why stiffness constant is not correct and why the other 2 are correct?
(edited 1 month ago)
I found this website that was presumably doing the same paper as you: https://www.cyberphysics.co.uk/Q&A/KS5/materials/A1.html
they say the "properties need to relate to the actual material not the form it takes", so I presume what theyre trying to say is that tensile stress and strain and spring constant k all depend on the amount of extension of the material, while the breaking stress and youngs modulus dont as they are fixed values
You could make springs of different stiffness (spring constant) out of the same metal by changing the shape and size of the springs.
So that why spring constant is not a material property.

Breaking stress for all samples of the same material is the same. If you doubled the cross sectional area, the force required to break it would be doubled... But the breaking stress would be the same. That's why its a material property.
How do we know that changing the shape and size of the spring will change the springs constant?
As k is directly proportional to A/L
Area of material/Length
So if L doubles k halves and if A doubles, k doubles
I think you derive this equation in a PAG
Original post by Gcsestudent56
How do we know that changing the shape and size of the spring will change the springs constant?

IIRC our teacher did a demo joining 2 identical springs in series to create the equivalent of one long spring with half the spring constant of the originals.

the even more convincing demo would be starting with a long spring and cutting it's length down and seeing what the effect on the spring constant is... but cutting springs is quite risky because bits of sharp metal might fly off at high speed so it's probably not going to be suitable for a classroom demo.
Original post by mosaurlodon
As k is directly proportional to A/L
Area of material/Length
So if L doubles k halves and if A doubles, k doubles
I think you derive this equation in a PAG

PAG?
Original post by Gcsestudent56
I ticked Young's modulus and stiffness constant. Answer was breaking stress and youngs's modulus. Can someone explain why stiffness constant is not correct and why the other 2 are correct?

k tells you how many Newtons are needed to give an extension of 1m.
This depends upon the dimensions of the spring and the material.
e.g. make a spring from very thin steel wire and another from the same material but very thick wire, all other dimensions equal.
The thick wire will require more force to extends it the same distance as the thin.
Original post by Gcsestudent56
PAG?
It’s like a practical but in a level that you get graded on