# What makes a spring stiff?

Does anybody know why a spring has a stiffness, does anyone know the physics behind it like the intermoleculaor bonds etc. This would be a great help as my coursework has to be handed in next week. Thanks

Mapes
Homework Helper
Gold Member
Hi fatboy_1989, welcome to PF. The atoms in any solid are spaced at an equilibrium distance from each other, at the balancing point between attraction and repulsion. (The attraction is electrostatic--electrons attracted to another atom's protons--and the repulsion is caused by the exclusion principle, if you've studied that.) The equilibrium spacing is the lowest-energy configuration, so it takes energy to stretch or compress it. When you stretch a spring, the work you are doing adds this energy.

thanks a lot very very appreciated

all materials have its stiffness. the strength of stiffness depends on the extension of the material under a definite amount of force ... the lower the extension, the stiffer the material is... in other words, it depends on the young modulus of spring, which is usally made of iron.
for forces between metallic atoms, it is usally consists of two kind forces - one with short-range repulsive force and that long-range attractive force.

for details, you can search on the internet or reference books.

Stiffness comes in many flavors. "Axial stiffness" $AE/L$ or "Bending Stiffness" $E I / L$, Torsional or Twisting $J G/ L$... The list goes on... By definition (as related to the solution of most problems) the stiffness is the amount of force it takes to deflect something (usually a solid) by 1 unit.