Gravity is the weakest of the fundamental interactions (see Section 7. 6).

(a) This being so, what makes it such an important force on Earth?

(b) What makes it the dominant force in galaxies?

Data from Section 7. 6

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An interaction is fundamental if it cannot be explained in terms of other interactions. The interaction between, say, two colliding carts is not fundamental because it can be explained as being the result of the interactions between the atoms that make up the carts. Nor are the atomic interactions fundamental because they are the result of the interactions between the electrons and atomic nuclei that make up the atoms. And inside each atomic nucleus we observe yet another interaction responsible for holding the components of the nucleus together. This last interaction, the strong interaction, is today classified as fundamental because it cannot (yet) be explained in terms of other interactions.