If in physics two quantities that are observed by different means turn out to be equal, then this equality should not be interpreted as pure chance. As a rule, the reason for it is a physical law in its own right. Some significant examples will be given in the following.
Our first example is the coincidence that inertial and gravitational mass, two different concepts, are exactly equal as shown by careful measurements. To discuss the difference between these two masses, we consider an object that is so small that we can consider it to be a point mass. Its inertial mass appears in the inertial force , which acts as resistance against the acceleration . The gravitational mass of the object appears in the gravitational force that the object exerts on other masses.
Isaac Newton (1643–1727) recognized the identity of and...
