How did Newton conclude a body in motion will remain in motion (unless acted upon by an outside force)? Since there are no frictionless surfaces and he had never been in outer space, we must conclude he took a leap of faith here. But, something must have given him the confidence to conclude this definitively. Maybe the SD knows…the rest of the story?
Newton based a lot of his conclusions on astronomical observations. Newton’s laws are in fact a generalization of Kepler’s laws, and Kepler developed his laws through very careful collection of astronomical data and extrapolation.
Actually, it’s probably more accurate to say that Newton’s laws of motion plus his laws of gravitation are a generalization of Kepler’s observations.
Newton also knew of Galileo’s thought experiments, including the frictionless plane.
Newton’s Body In Motion sounds like a workout video for physicists.
Yeah, something constant and low-impact, with a lot of translations and rotations.
I need a “Like” button for the above image of “Newton-robics”!
Why low-impact? I think inelastic collisions are what we’re after, here; something to absorb energy.
The OP’s idea is not very smart. Newton could of course roll a marble across a window pane. Therefore he had a very low friction surface, and he could measure the slope and estimate how slope affected it and so on…
No. Its rather hard to jump from Kepler’s Law to Newton’s laws.
Hooke (as in Hooke’s law of springs/elastic strain…force is proportional to distance stretched.) also contributed by proposing that the centripedal force that obeyed a square law with distance would be a good start to explain the orbits… and obey Kepler’s law and match the observations.
But it was Newton who used his own ability with calculus (which he and Liebnitz contributed to.) to work out that that his theory of gravity worked out quite well with both the planets and with things being thrown, eg arrows and pushed down here on earth…
What Newton published was his reduction … the laws that he boiled out of all he did. I don’t think that trivial experiments down here on earth were the important thing… Newton knew that people would have all sorts of explanations for why he was wrong … “but arrows make a loud noise, its the noise that makes the arrow slow down !”. and so on… But the planets in orbit… well thats very accurate measurement and totally repeatable … if every planet and moon obeys the law with the SAME gravitational constant… then it must be the correct laws… how could moons around different planets obey the same laws down to the G in use ?
Newton didn’t have the capability to prove that every planet and moon obeys the law with the same constant. In fact, even we don’t have that capability. To do so, you need to know the relative masses of all of the gravitating bodies, and the only way we have to find those is to assume that they use the same G.
Nobody said it was easy. That’s why it took a guy like Newton. A large portion of his Principia discusses Kepler’s empirical observations of the solar system and then shows how Kepler’s laws are a specific application of Newton’s more general formulation of motion and gravitation.
The idea about inertia allowing an object to keep moving absent any forces was actually developed by Galileo. The thought process is actually pretty simple.
Imagine placing a ball on an incline. It starts slow, then speeds up as it rolls down.
Now give a ball an initial push up an incline. It starts fast, but slows down to a stop as it rolls up.
So a ball will speed up when rolling down, and slow down when rolling up. If the ball is rolling along a flat, level surface, then it is neither rolling up nor down, therefore it should neither speed up nor slow down, and remain the same speed. Of course, this only holds if gravity is the only thing acting on it. But Galileo saw that the smoother the surface, the longer an object will keep moving along it, and extrapolated that a perfectly smooth surface will allow an object to keep moving forever.
Newton realized that this idea could be applied to the planets in their orbits. Since there is no friction in space, there is nothing to slow the planets down, so they can keep moving forever without anything continually pushing on them.
The only difference was that Galileo assumed that the natural motion, absent any net force, was always a circle (including, on the surface of the Earth, a great circle about the planet), not a straight line.
Is this what you meant?
Galileo did a mess of work on motion … before he got his hands on the telescope … he had some flawed ideas but he was very careful recording his data … which he was able to compile during his years of house arrest … so Newton was aware of some kind of relationship between motion and time …