## Chapter 12

## Motion of a System of Particles. Rigid Body Rotating about a Fixed Axis

Newton's second law of motion, in its purest form, applies to particles. It leads to diffrential equations by solving which we can obtain the detailed motion (including the trajectory) of a *particle* moving under well specified external forces. However, all material objects are “extended objects” containing a large number of particles. Let *N* represent this number. If we consider an atom to be a “particle” then for a 1 kg stone *N* ~ *N*_{A}, where *N*_{A}=6.02 × 10^{23} is Avogadro number. Can we use Newton's Laws of motion to such *Extended Material Objects* (EMOs)?

We touched upon this point at the beginning of Chapter 5 (on page 100). There we laid out the following principle: ...