Mass

• Is the Planck Mass About the Mass of One Eyebrow Hair?
  The Planck mass is about 22 micrograms—comparable to a small hair, but the match depends strongly on a hair’s length and thickness.
• Density vs. Mass Density: What Is the Difference?
  Density usually means mass per unit volume in basic physics, but the term also applies to other quantities; context and units clarify which one.
• Can the Mass of a Body Change?
  A body's mass can change when matter or energy enters or leaves it. Learn how classical mass, rest mass, and mass–energy equivalence differ.