Couple questions regarding Newton: 1) Sort of a historical question...In Newton's time, how did Newton explain the stopping of an object due to friction? I imagine that would have been the first objection to his first law. Could he have said that the earth's momentum had changed due to the stopping of the object? Could his detractors not have said that such a change in the earth's momentum cannot be measured? I am merely curious about how easily his laws, especially in regards to interactions with the earth (where one side of the equation cannot be measured) went over with his contemporaries. I am looking at this purely from a conservation of momentum standpoint, rather than conservation of energy. 2) I often read that Newton's Second Law should be expressed as F = dp/dt rather than F = ma. When mass is treated as a constant (and concerning external forces, it should be), what's the simplest example of a calculation where using F = ma gives the wrong answer (rather than be less convenient), if any? Or is it just that Newton's point in his third law was in regards to momentum, and to speak of momentum rather than acceleration in his second law would be more consistent?