III.

Dynamics

To understand why and how objects accelerate, force and mass must be defined. At the intuitive level, a force is just a push or a pull. It can be measured in terms of either of two effects. A force can either distort something, such as a spring, or accelerate an object. The first effect can be used in the calibration of a spring scale, which can in turn be used to measure the strength of other forces: the greater the force F, the greater the stretch x. For many springs, over a limited range, the stretch is proportional to the force

F = kx

where k is a constant that depends on the material and dimensions of the spring.