And this is proportional to the mass, So that if we were to take two different objects, first swing one, and then swing another one at the same speed around the head and measure the force in the second one , that second one, the new force is bigger than the other force in the proportion that the masses are different. This is a way of measuring the masses by how hard it is to change the speed. Now then Newton saw from this, that for instance, to take a simple example, if a planet is going in a circle around the sun no force is needed to make it go sideways, tangentially if there were no force at all on it, it would have just keep coasting this way. But actually, the planet doesn't keep coasting this way, but finds itself later not out here, Where it would go if there were no force at all. But further down toward the sun. In other words, its velocity, its motion has been deflected toward the sun. So what the angels have to do is to beat their wings in toward the s...