Actually, the boy pulls on the rock (centripetal force) and the rock pulls on the boy (centrifugal force). The string is the medium through which these equilibrants do battle.
Originally Posted by golf_sceptic
Thanks for replying yoda.
Ok. There's a hair which may need to be split depending on whether the string's mass can be ignored, but so far so good.
Are we agreed that centrifugal force does affect the boy but in any way affects the rock?
While we are hair splitting - are they both equilbrants or is one a resultant?
While we are hair splitting - are they both equilbrants or is one a resultant?
I'm not familiar with the term "equilbrants", but action/reaction pairs are always equal and opposite, and it is helpful to identify the action/reaction pairs before considering resultant forces on each object.
I would consider equilibrium (taking a guess at your meaning) to apply to an individual object within a system, or to the system as a whole. Action/reaction is a different concept to equilibrium.
I'm not familiar with the term "equilbrants", but action/reaction pairs are always equal and opposite, and it is helpful to identify the action/reaction pairs before considering resultant forces on each object.
I would consider equilibrium (taking a guess at your meaning) to apply to an individual object within a system, or to the system as a whole. Action/reaction is a different concept to equilibrium.
Sorry about that. My mistake. I am in well over my depth here but I am trying to understand what you are getting at.
Do the image and definitions have any bearing on this topic?
Resultant - the single force that is equal to 2 or more forces.
Equilibrant - the single force that will balance 2 or more forces. Equal in magnitude but opposite in direction of the resultant.
Do the image and definitions have any bearing on this topic?
Yes. In terms of your diagram the main idea I'd like to add is that in general the three forces are all acting on the same body, but will come from different action/reaction pairs.
Are we agreed that centrifugal force does affect the boy but in any way affects the rock?
Actually, the rock is simply trying to move in a straight line. The centripetal string prevents it from doing so, and the apparent force -- from the rock's point of view -- is centrifugal.
Actually, the rock is simply trying to move in a straight line. The centripetal string prevents it from doing so, and the apparent force -- from the rock's point of view -- is centrifugal.
Excellent, we've got to the core of the issue very quickly.
When we speak of the rock's point of view, we are talking (I hope) about what you would see through a tiny camera attached to the rock. Through this camera we would see such interesting sights as a stationary rock (because we are attached to it), the centre of motion (the boy's hand) would be stationary, and everything else in the world would appear to be in the process of being flung from away from the centre of motion. The centrifugal force (away from the centre) will neatly balance the centripetal force (toward the centre), but no net force is acting on the rock. It is not being flung out. It just sits there stationary in the camera shot.
The problem is that this is not a helpful frame of reference because it is not an inertial frame of reference. We have to do all sorts of fudging if we are to explain why all of the objects (grass, trees, etc) are being flung away from the centre.
If we want to apply the usual laws of physics we need to look from an intertial frame of reference. The string pulls in on the rock (centripetal). The string pulls out on the man (centrifugal). The force on the rock is not balanced, and the centrifugal force only affects the string and the boy.
Excellent, we've got to the core of the issue very quickly.
When we speak of the rock's point of view, we are talking (I hope) about what you would see through a tiny camera attached to the rock. Through this camera we would see such interesting sights as a stationary rock (because we are attached to it), the centre of motion (the boy's hand) would be stationary, and everything else in the world would appear to be in the process of being flung from away from the centre of motion. The centrifugal force (away from the centre) will neatly balance the centripetal force (toward the centre), but no net force is acting on the rock. It is not being flung out. It just sits there stationary in the camera shot.
The problem is that this is not a helpful frame of reference because it is not an inertial frame of reference. We have to do all sorts of fudging if we are to explain why all of the objects (grass, trees, etc) are being flung away from the centre.
If we want to apply the usual laws of physics we need to look from an intertial frame of reference. The string pulls in on the rock (centripetal). The string pulls out on the man (centrifugal). The force on the rock is not balanced, and the centrifugal force only affects the string and the boy.
The frame of reference is always a choice. The laws of physics in this case are invariant regardless of frame of reference. However, the explanation of what is going on varies. A rotating reference frame is proper during the downswing since that is the frame of reference the golfer lives in so to speak. You wish to keep things in a non-rotating reference frame because it makes the centrifugal force explanation less applicable.
This whole argument is splitting hairs about reference frames and centrifugal force. It is pointless to the golfer. One will get the right answer whether one invokes the concept of centrifugal force or explains the phenomena as inertial resistance of the clubhead mass. Basically it all comes down to what folks will understand more easily. Centrifugal force explanations are more intuitive to understand for the majority than inertia which is why it is more generally taught that way in universities across the world (to physicists). It is nice that you learned your physics a different way, but the answers all turn out the same. I suspect we will just have to agree to diagree, Jack.
__________________ _________________________________
Steph
Distance is Magic; Precision is Practice.
The laws of physics in this case are invariant regardless of frame of reference.
Ok. Let's try that assertion out with a question. Why in the frame of reference of the rock does the centrifugal force act on the rock when in an inertial frame the centrifugal force acts on the boy?
A non-inertial frame of reference changes everything. Forces appear like the ones which the camera on the rock shows affecting the trees and the grass. It becomes a mess. The simple reason I wish to keep things in a non-rotating reference frame is because physics in a rotating frame is so complicated, but will after a lot of hard work provide the same answers (for the ball) as in an inertial frame.
Newton's law that "an object at rest will remain at rest unless a resultant force is applied" becomes "an object at rest will only remain at rest if a centripetal force is applied". See what I mean?
I have no problem if you wish to disagree, but be so kind as to point out the error in what I say rather than discussing motives and making condecending remarks like "it's nice that you learned your physics in a different way".
I will, however, be very interested in reading your explanation of the physics of the throw out effect in a non-inertial frame of reference if you want to discuss it.
Last edited by golf_sceptic : 06-28-2006 at 09:19 AM.
1-H (last paragraph) "As a term is specifically defined herein, that is the basic connotation which is always a dictionary definition but not necessarily that of Physics, Electrical, etc. The dictionary is generally considered a standard of precision. Scientific terms in quotes denote a loose application with obvious intent, because no better term seems available. Measurements given herein are for the golf course rather than the laboratory, but the laboratory will show them within acceptable tolerances. Clarity and usefulness are the only motive."
At the time of Homer Kelley's death in 1983, Merriam-Webster's New Collegiate Dictionary defined the following:
centrifugal force n: the force that tends to impel a thing or parts of a thing outward from the center of rotation
Hybrid Mode
