Physicists have three different definitions of mass, all thought to be equivalent. Measurements of the distance between Earth and the moon confirm that two of those masses are one and the same to higher precision than ever before, physicists report July 13 in Physical Review Letters. That result confirms one of the most basic foundations of physics: Newton’s third law of motion.
Inertial mass determines how easily an object accelerates in response to a force. Then there’s active gravitational mass, which determines the strength of an object’s gravitational field. Rounding out the trio is passive gravitational mass, which dictates the force on an object in a given gravitational field.