Originally Posted By: onion
Semantics and convention. Ya'll have only demonstrated why the convention you use is preferable- and I'm not arguing that point. It IS more convenient.
But none of ya'll have explained away the centrifugal force on 'point A' in my tether-ball example. It's exerted BY the ball. It opposes centripetal force from the string. It's exactly what people are describing when they talk about "centrifugal force".
Like I've said too many times: You may not like the frame of reference I'm using. It may well be mathematically inconvenient. But that force exists, and ya'll haven't shown otherwise.
This isn't a discussion on semantics, it's on principles. And the mathematical convenience is not the issue (we use constructs like massless strings to make the math easier for the noobs...). We've got to get a shared undertanding of the basics to move on and explain the questions you're asking. Actually, it's Pseudo Forces that make the math easy, that's why engineers use them for calculations of things like drag...but we're not to that point yet. We have to get to an understanding of the three laws of motion and pseudo forces, which I have already explained in detail. And you can't just choose a new reference frame without understanding what takes place in every reference frame. So let's drop the reference frame consideration for now, it's complicating the discussion.
The definitions are central to understanding. If you really want to understand the physical world, it's physics that describes it, not comforting notions of perception, and you have to use the terms correctly or we're talking past each other...I didn't take this out of the layman's realm, but we are out of the layman's realm now. If we are more comfortable back there, then yes, it's turtles all the way down and centrifugal force tore the turbine apart...if that's not the path y'all want, then read on...
When I have a bit more time, I'll explain how one applied force (not tension, but the centripetal acceleration) can tear a turbine apart. Maybe we'll even get to how an airplane works and why the engineers that design them choose a non-inertial reference frame (hint: it's to make the math EASIER...physics is hard, even engineers struggle with it sometimes...) Look, Physics isn't easy, I had 2 years of Newtonian mechanics in High School, then majored in Physics in college...it took me years to gain the understanding that I am trying to impart (with admittedly little success...) here.
For now, let's get to Newton's third law of motion as we proceed down the path of physics.
You're "feeling" a force when you swing the ball, but what you're feeling is not the ball pulling...it's YOU pulling the ball...I never got to Newton's 3rd law (for every action there is an equal and opposite reaction). The 3rd law is one that is confusing to most folks.
The tether ball is a great example - everyone keeps saying "who cares where it goes if the string breaks?" but that is the quintessence of the question. The ball only moves because it is being PULLED by the string. It goes in a circle because it is in accelerated (circular) motion constantly. If the string breaks, then we're back to Newton's first law - the ball moves in an inertial path.
If there are two equal and opposite forces on an object, (remember that forces are vectors, not scalars, they have direction as well...) then they cancel each other out. There is no NET force. Since a force, by definition, imparts an acceleration, if you had two, equal and opposite forces (centripetal and centrifugal), then there would be NO acceleration towards the center and the ball would proceed on an inertial path. So, there simply can't be an opposite (or equivalent) force. There are only pseudo forces - those things that satisfy human perception. In order to impart a force, we have to keep in mind Newton's third law (for every action there is an equal and opposite reaction). The ball isn't "pulling" on me - I am imparting a force to it. Not semantically important but critically important for understanding.
So, forget for a moment that the ball is going in a circle. Picture a rock on a hockey rink (as close to frictionless as we can get..). Tie a string to it (strong, but zero weight, so we can ignore the force that you have to impart to the string itself and focus on the rock). You pull the string - the rock accelerates. Its velocity changes only as long as you pull on the string. Once you stop pulling the rock is sliding with a direction and a speed. Newton's first law...it will stay sliding (again, work with me, this is a frictionless rink...we all know that friction is not zero even on a rink...).
If you want to change that direction and speed - you pull on the string, could be same direction, doesn't have to be, and we're back to Newton's Second law - you impart a force - you change the rock's velocity. Your force is measured as a function of mass (the rock) times acceleration (change in rock's velocity). you don't have to pull the string in the same direction, it can be in any direction and you are imparting a force. Definition of a force was covered in my first lesson.
If you pulled on the string very gently (small F), then the A will be very small too...that's how ion thrusters on satellites work, micro newtons of thrust (very small F, very very small A) - over a long time and you get big changes in V, but that's impulse (force over time) and we're not ready to get to that yet.
In fact, here's one of the interesting things - I can accomplish that same acceleration using Newton's third law. This is how rockets work. Before the first rockets flew into space, engineers and scientist passionately argued that they could not fly without something to "push" against. We know that's not true, but the argument that the rocket has to "push" against the air is precisely the same argument as the equivalent force and the reality of pseudo forces.
But the third law is more elegant and simple than the pseudo force argument. Rocket propellant goes out the back of the rocket. The rocket moves the other way. Energy must be conserved. We can do this with a big rock on our hockey rink. If I stand on that big rock and throw a Brick, then rock and I move the opposite direction (3rd law). If I have a lot of bricks, and Roger Clement's arm... I can keep throwing them and the rock and I will acclerate in the opposite direction because I imparted a force on that thrown rock and the opposite reaction force is imparted on me. If I can keep this up...at some point, the rock will be moving faster than the bricks I am throwing, each brick MUST accelerate the rock (3rd law and conservation). That's how Apollo got to the moon - the rocket exhaust was about 5-10,000 MPH, but the spacecraft ended up going about 25,000 MPH - escape velocity for lunar insertion.
There is no "equivalent force" for my brick throwing or rocket exhaust. There can't be, if there was something acting on the rocket besides what I have described, it wouldn't work...it wouldn't accelerate enough to get to the moon (or even into orbit). And there is no drag in space to complicate the math or principles...there is only the 2nd law, F=MA, 3rd law, and conservation.
And they work perfectly and completely to describe the motion.
Here ends the seventh lesson in Physics.
