Galileo’s two major contributions to modern physics were the ‘Law of Falling Bodies’ and the ‘Law of Inertia’. Inertial motion of an object means that this motion can solely be changed by forces acting upon it. Gravity accelerates all objects equally regardless of their masses or the materials from which they are made and this is a cornerstone of modern physics. Galileo discovered the law of inertia, but he did not name it. Newton wrote about inertia in his book the ‘Principia’ saying, “A body, from the inert state of matter, is not without difficulty put out of its state of rest or motion.” Gravity unlike magnetism, treats all materials and circumstances alike. Very surprisingly, its force is proportional to the same property, mass, that characterizes a body’s inertia. Modern physicists instead often distinguish two kinds of mass, gravitational and inertial, which are said to be “equivalent”.
The equivalence principle of masses is attributed to Newton as he conducted experiments and performed tests. This principle helped explain the question of why all bodies fall to the ground with the same acceleration. This is supposedly what Galileo discovered when he dropped objects of different weights from the Tower of Pisa (or rolled balls on ramps). Newton explained this phenomenon by introducing the concept of mass, a new quality of matter. In Newton’s laws, mass acts at some times as a measure of inertia and at other times as a measure of gravitational properties.
The equivalence principle became a fundamental law of physics stating that gravitational and inertial forces are of a similar nature and often indistinguishable. In the Newtonian form it asserts, in effect, that, within a windowless laboratory freely falling in a uniform gravitational field, experimenters would be unaware that the laboratory is in a state of non-uniform motion. The equivalence principle is at the base of most known viable theories of gravity. In Einstein’s version, the principle asserts that in free-fall the effect of gravity is totally abolished in all possible experiments and general relativity reduces to special relativity, as in the inertial state.
In 1971, Dave Scott an astronaut from Apollo 15 was standing on the Moon, when he held his hands out at
shoulder height, with a hammer in one hand and a feather in the other. The world looked on via live television as he let go. The feather didn’t drift to the ground, it plummeted, falling just as fast as the hammer. Without air resistance to slow the feather, the two objects hit moon dust at the same instant, proving that Galileo was right.