Pendula are not the only oscillating system. Another approximation for many systems is a mass hanging on a spring. Although the forces here are a bit easier to analyse, we do need to make the assumption that the mass and spring system is in space!
This is another example of simple harmonic motion, as the acceleration is proportional to displacement and acting in the opposite direction.
We know that, for springs obeying Hooke's law, the force is proportional to extension. For a spring in space, the elastic force is the resultant force. The acceleration of a mass hanging from a spring is therefore proportional to its displacement from the equilibrium position.
Use quizzes to practise application of theory.