Others have already given the modern answer, but the historical perspective and how it was discovered is also interesting. It's not obvious (some people thought

Others have already given the modern answer, but the historical perspective and how it was discovered is also interesting. It's not obvious (some people thought that m|v| is conserved) but that those two are conserved can be deduced from intuitive facts about mechanics that you already know, plus the fact that gravitational acceleration is constant. Christiaan Huygens, who predates Newton, analyses the situation in this very interesting article, which Newton cites as an influence when he presents his laws: http://www.princeton.edu/~hos/Mahoney/texts/huygens/impact/huyimpct.html http://www.princeton.edu/~hos/Mahoney/texts/huygens/impact/h...