Complex sentence: For everyday objects and energies well-described by Newtonian physics, mass has also been said to represent an amount of matter, but this view breaks down, for example, at very high speeds or for subatomic particles.
Split into simpler sentences: For everyday objects and energies well-described by Newtonian physics, mass has also been said to represent an amount of matter. But this view breaks down at very high speeds or for subatomic particles.