 In Newtonian physics, a typical physical object, such as a billiard ball, has associated with it a positive, real number called its mass, for which the symbol \(m\) is commonly used. When we are focusing on describing mathematically the motion of this object under the action of contact forces, such as when one tries to predict where a billiard ball will go after a collision, the mass in question is also called the inertial mass. In the special case in which one considers how the billiard ball moves in response to gravity, say as it falls toward the earth, the measure of how the billiard ball “responds” to the gravitational field is called the gravitational mass of the object. Using experiments in which he filled wooden boxes with different materials and suspended them from strings to construct pendulums, Newton discovered that inertial mass and gravitational mass are directly proportional. Physicists have since customarily treated inertial and gravitational mass as numerically equal. 