Isaac Newton was the first who first explained gravity from basic principles like Newton's law ``` m x'' = F(x) ``` relates the mass, the acceleration x'' with the force. The force from a body with mass M whose center is in in distance x one has the inverse square law F(x) = -M/x2. In the case if x is large, then by linearization one can assume a constant force like F(x) = <0,0,-10> like on the surface of our earth. In that case, there are solution x(t) = x(0) + t x'(0) - 5 t2. This solves the free fall problem. Later in this course, we rewrite the law of gravity as ```div(F)(x) = rho(x), ``` where rho is the mass density. This leads to the inverse square law if rho(x) is rotationally symmetric. General relativity refines the law of gravity even more and explains it geometrically. We still do not know what mass is. The search for the Higgs boson might provide answers in the near future. Ali G talks to Jerome Friedman from MIT about gravity. Friedman gave the first experimental evidence that protons consist of quarks. Source: Youtube.