Bringing the apple to the Moon

Newtonian gravity can be regarded as a hypothetic-deductive system where the inverse square law is the starting point from which gravitational phenomena are deduced. This operational form of presenting gravity endorses problem solving and seems to be predominant in the teaching practice. In contrast, regarding phenomena as a source for the development of the theory is also possible, of course, and can be advantageous to scientific education since it deals with model conception and construction. This article intends to introduce undergraduates to Newtonian gravity using its empirical basis, i.e. the free fall and the planetary motion, to deduce the universal law of gravitation. It also steps into the modern interpretation of gravitational phenomena i.e. Einstein’s general relativity, including a discussion on the instantaneous action at a distance in this context. This didactic presentation of the Newtonian theory of gravity is designed to reach a threefold equality, similar to those applied in the method of separation of variables in partial differential equations, where G is treated as a separation constant. By doing so, the universality of the gravitation constant emerges as a conclusion rather than a statement. A few historical remarks on the development of this topic are also highlighted.