Hooke's law — the elastic force grows in proportion to the deformation.
Symbols
F
elastic force
[N]
k
spring constant
[N/m]
x
extension (deformation)
[m]
Where it comes from
A spring exerts a force that grows the more you deform it — in proportion to the extension. The force is restoring: directed opposite to the deformation.
When to use it
Springs, spring balances, oscillations — up to the material's elastic limit.
Example
A spring with constant k=200 N/m is stretched by 0.1 m. Find the elastic force.