Euler's first law, which defines linear momentum as the product of mass and velocity of a body's center of mass, primarily applies to single bodies. However, the discussion raises the question of whether it can be extended to systems of bodies or if conservation of momentum should be used instead. Internal forces within a system do not affect the total momentum, suggesting that Euler's law can be relevant for collections of bodies. The distinction between Newton's laws, which focus on particles, and Euler's laws, which apply to bodies, is emphasized. Ultimately, the conversation highlights the complexity of applying these principles to systems of multiple bodies.