Sir Isaac Newton was wrong about pretty much everything about which he was right. His calculus was brilliant, but his crazy notation for it set English mathematics back decades if not centuries. His theory of gravity is sound enough, but only as an abstraction. And his Laws of Motion were revised by Euler and then made quaint by relativity. And yet we keep talking about him. Why? Because, like most things, unless you're a purist, a rough abstraction which accurately describes the situations in which you find yourself is good enough. Hell, even purists have to deal with differential equations, which are one hand-wave away from being well-informed guesses.
I don't have much patience for the arcane if it can't be made plain, and even Newton can be arcane, so let's see if we can't demystify him slightly.
Via Wikipedia:
Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.
Law II: The alteration of motion is ever proportional to the motive force impress'd; and is made in the direction of the right line in which that force is impress'd.
Law III: To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.
Yeah.
In the first law, everyone says, 'A body at rest tends to remain at rest, and a body in motion tends to remain in motion.' Which gives the impression that he meant 'tends to' (which he never actually said) to mean that an object wants to remain in some way. Objects don't want. The universe doesn't want. "Tends" in this case simply means "has a tendency to," which means "will, all things being equal."
So, rather, let us say that an object's state of motion, either at rest or moving in a straight line, doesn't change without something making that change. If something is moving, it doesn't slow down or speed up without something making it slow down or speed up, and if something is still, it doesn't start moving without something making it move. Seriously, it took all of human history up to Newton to write that down. People are idiots. It seems screamingly obvious when you look at it; things don't change unless something changes them.
But it holds a key idea: inertia. The thing is, what is it that makes objects stay in their state of motion? It's not a desire, either on the part of the object or the universe. It's a property of mass, an energy that mass has. Crazily enough, no matter how fast an object is going, its inertia never changes; therefore an object always requires the same amount of force to accelerate the same amount. But we'll get to that in a minute.
People also use "inertia" to mean "momentum," which it isn't. An object in motion has momentum, which increases proportional to both mass and speed. A car with half the mass of another car travelling at twice the speed will have the same momentum and would exert the same force when it hit a wall. But it would take 1/2 the force to accelerate the car with half the mass, no matter how fast or slow the two were going.
Which leads directly to our second law, which is typically expressed as a math equation, F=ma. Or rather, it should be a = F/m, as if we were saying that "alteration is proportional to the Force motive," which is what Newton said. But what it really means is that acceleration is proportional to force as it acts on mass.
What does it really mean? It means that if you push something hard, you'll accelerate it faster than if you push it more softly.
When I say it like that, again, you're thinking, "What the hell, rest of history leading up to Isaac Newton?" But it's an important thing to know; if it weren't true we'd be like really bad golfers who, when they hit the ball, have no idea how far it will go. There would be no correlation between the amount of force applied to an object and the change in its motion.
Newton also adds the part that most people take for granted or don't understand; if you push an object in a direction, it'll accelerate in that direction. Again, no big news there, but that means that, in our crazy world of vectors (that's a quantity which has a direction) rather than scalars (which is just a quantity) we don't have to worry about throwing a ball and having it go off at a right angle to the direction we threw it (unless we're really bad at throwing balls, but that's not a violation of the basic laws of physics).
The third law is the trickiest and the easiest to wave your hands at and explain away, and yet it gets quoted a lot, particularly in places where it doesn't belong. If you're using Newton's Third Law of Motion to describe love, or politics, or really anything but Newtonian physics, you're doing it wrong. In Newtonian physics, however, for every action there is an equal and oposite reaction.
Why? That's pretty silly if you think about it; that means that if I push against a wall, the wall pushes against me. if I throw a ball, the ball throws me. In Soviet Russia, car drives you. What, Newton was doing Yakov Smirnov?
Let's break it down here; no need to consider any force but pushing, because everything follows from that. So if you apply force to an object, the object applies force back. It's almost relativistic. Consider; if you push against a wall, does it feel any different than having the wall push against you? It does because your muscles are exerting that force, so you feel them working. But supposing you have to hold something up. Suppose the wall was about to fall on you. The only way you'd be able to keep the wall in place is to push back with the same force as it's pushing against you (gravity is a force; the "Force of Gravity" isn't about the Jedi).
That push-back comes from inertia. Yes, if you push harder, the push-back is equal (and opposite, which means that it won't push sideways because vectors complicate things). But the acceleration caused by that force will be dictated by inertia. So, in The MILP™, if you push against an object with exactly the same mass as yourself, both you and it will move away from each other with equal acceleration and wind up traveling at the same speed in opposite directions (let's leave relativistic motion out of this for the moment). If you push an object half your mass, it will move away at twice the acceleration as you and wind up traveling at twice the speed you do (but, in that perfect ideal, you'll still wind up traveling at the same speed as in the first example. Why? Because while the forces were equal, the inertia wasn't, which means that the same force acting on half the mass resulted in twice the acceleration.
That's the big takeaway from Newton, actually: objects have inertia. You could, I suppose, phrase all three laws as just that. But then you'd have to define "inertia' and you'd wind up saying, "Well, inertia means that objects at rest tend to stay at rest..." and you'd wind up defining inertia by Newton's Laws of Motion.
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