- A-Level Physics

# Materials

Materials can have many different properties, and experience a number of external and internal forces. The two main such forces are the** tensile and compressive forces**, relating to extension (stretching) and compression (squishing) respectively.

## Hooke's Law

The most common example of these forces being applied are in a helical spring: these have a natural length, and are either stretched or compressed depending on direction of force applied. The distance of this deformation is called the extension (regardless of direction) and according to **Hooke's law**:

Extension is directly proportional to the force applied, provided the elastic limit is not passed

As you can see in the graph, **the relationship is linear**, where the **gradient is the force constant** up to the elastic limit - if this is exceeded, the object will no longer return to its initial shape, - **plastic deformation **has occurred.

F = kx Force = force constant x extension

### Multiple String Systems

Springs (and indeed any elastic materials) can be combined either in series or in parallel, with vastly different effects:

## Elastic Potential Energy

All springs, just like any object under tensile or compressive forces, store Elastic Potential Energy. This is equivalent to the work done in extending/compressing them, and since W = Fx, this is the area under a force-extension graph.

W = 1/2 Fx E = 1/2 Fx E = 1/2 k x^2