# Rest Mass

My book says that if two objects collide completely inelastically, the rest mass of the composite is greater than the sum. I assume this happens because the objects heat up and this is counted in their energy.

However, in other problems the book assumes the rest mass of objects/particles is the same before and after collisions. Explanations are sparse.

How do you know if the rest mass changes or not?

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BruceW
Homework Helper
I am guessing this is a book on relativity? What mathematical methods have you been through so far? Have you learned how to calculate the four-momentum of a system yet? and how to get the rest mass from this? It would be best to understand it mathematically, but I could help with an intuitive explanation if you haven't been through the maths yet.

End of a book on mechanics. The next chapter is more formal. I would just like an intuitive grasp of what the rest mass is and how to set up simple conservation law equations such as in compton scattering.

BruceW
Homework Helper
hmm. I could write out a big paragraph explaining 4-momentum as well as I can, but I don't think the homework section is the right place for that kind of thing. But I can help with the confusion you had in your first post. Right, so the rest mass of two particles is not just the sum of their individual rest masses. I think this is where the confusion stemmed from.

Edit: Also, when several particles are involved, it is often called the 'invariant mass' of the system, and is usually just called 'rest mass' when only one particle is involved.

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