Work done on a rigid body in a collision

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It applies wherever Newton's laws hold, which I would not describe as having limited utility.
It's a useful theorem, but different than conservation of energy.

I'm not talking about conservation of energy. I'm talking about a version of a work-kinetic energy theorem (if it exists in enlish language) which is valid for systems of particles and, as a special case of this, for rigid bodies (that is a system of particles where the mutual distances between them is constant, so the work done by internal forces is zero).

So it's easy to prove (matt helped me in this) that, for a rigid body, the total work done by the external forces is equal to the kinetic energy variation of the body. Here "work" means "real work" and "kinetic energy" means "total kinetic energy" so even rotational kinetic energy.

For example In italian language is called "Teorema delle forze vive" or "Teorema dell'energia cinetica" for systems of particles or for rigid bodies as a special case; maybe in english language it has a different name than "work-kinetic energy theorem".

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[Doc Al]

I'm not talking about conservation of energy. I'm talking about a version of a work-kinetic energy theorem (if it exists in enlish language) which is valid for systems of particles and, as a special case of this, for rigid bodies (that is a system of particles where the mutual distances between them is constant, so the work done by internal forces is zero).

The theorem I described, which is usually called the work-energy theorem, is valid for systems of particles.