in 1684.

Newton's proof that any body moving under the influence of a force that always acts in the direction of a fixed point obeys Kepler's area law is illustrated in Figure 8.4. His method was to replace the continuously varying force with a series of equally spaced impulses. Suppose that the body at A moves inertially (i.e. in a straight line), but is acted upon at equal intervals of time by an impulse directed toward the fixed point S. In the first interval of time, the object moves from A to B and would continue to D in the second interval if left undisturbed. However, the action of the force toward S is to deflect the body to C where

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