Lu,
I note that you have started your own thread on this (so you can delete it?) despite several requests on the Aerodynamics – Phase Lag thread for you to address the issue there. I have re posted the information here for you.
It is important that you refer to the discussion below by its relevant number so we can look at it in isolation, thus we can find some common ground. As what is presented below is a logic chain (the next point requires understanding of the previous point), all you have to do is stop at the first point you do not understand, and discuss THAT POINT ALONE. Easy.
Simplified, here goes:
1. An object will remain at a steady state of motion (either rest or at a constant speed) unless a force is applied to it.
2. Speed can be thought of as directional. An acceleration is a force.
3. To increase speed in a direction, you must apply an acceleration in THAT SAME direction. Think of a car which accelerates. If the direction of the acceleration was depicted as an arrow, the pointy end of the arrow would be pointing at the front of the car, I.E. in the direction of the speed change.
4. THE LOGIC THUS FAR: Speed is directional. Speed change requires an acceleration (which is also directional). THUS changing direction requires an acceleration, which is depicted as an arrow pointing toward the direction in which the acceleration force is acting.
5. When a car turns a corner, it changes direction, therefore it MUST have an acceleration (or force) applied to it (remember the speed is directional) in the direction of the corner.
6. Which way do we draw the arrow to depict this force? The arrow MUST be drawn toward the centre of the turn because that is the direction the force is acting, just like we did for the car's straight line acceleration.
7. THE LOGIC THUS FAR: when turning, you are changing direction, and an acceleration can be seen to be acting toward the centre of the turn.
8. David's sling is wrapped around the rock, and the rock is traveling at speed. but the rock is not travelling in a constant direction - it is traveling in a circle. Thus the rock is constantly changing direction. Thus a constant force MUST be being applied to make it constantly change direction.
9. We can depict the force acting on the rock by drawing an arrow. Just as we did with the car, the pointy end MUST BE pointing in the direction that the force is acting, I.E. toward the centre, I.E. toward David's hand.
10. THE LOGIC THUS FAR: There is a force that can be seen to be acting toward the centre of the circle.
Physicists decided to give this force a name so that we could refer to it without saying "that force that acts on a mass that is changing it's direction"
They named it centripetal.
11. . When the sling releases the rock the force trying to change it's direction is gone, therefore the rock will travel in a straight line again, along a tangent. If the rock were to fly out of the sling in any other direction, an acceleration must be applied (because we know that a change of direction can only be accompanied by an acceleration).
Lu, you have mentioned several times the “force” that you feel is flinging objects outward through your marble example and your FAA question. I believe that that feeling of being thrown outwards is central to the difficulty in understanding centripetal, and the reason centrifugal is used as an easy substitute to promote understanding.
12. Going back to the logic steps I used above to describe centripetal, I used a car accelerating along a straight line as an example of a change of speed. Because speed had direction, and acceleration does too, when we depict the force, we can draw an arrow in the direction the car is traveling. in other words, to where the car is accelerating TO. Thats why centripetal was IN toward the centre of the circle: because it was an acceleration TO that direction.
13. So why do we feel flung outwards? When the car accelerates in a straight line do we feel a force? You betcha. We sink into the seat. We would get "flung backward" if the seat was not restraining us. In other words - we "feel" the acceleration in the opposite direction to which it is commonly thought of as being applied. In this case, the acceleration is going forwards, but we feel it going backwards.
14. Thus, when we are turning, we feel the opposite direction to the acceleration. I.E. we feel "centrifugal" because of the application of centripetal. Thus the force is centripetal (same as acceleration forward) and the sensation we feel is the REACTION (Thank you Mr Newton) to that force, but is NOT in itself a force.
15. THE LOGIC SO FAR: Centripetal acts like ALL acceleration forces – in the direction of that acceleration. When an acceleration force is applied, we “feel” it in the opposite direction but that “feel” is NOT a force of its own – it is a reaction to the acceleration.
16. SUMMARY: Centripetal acts toward the center. Centrifugal is NOT a force – it is the term applied to the reaction of centripetal.
I guess they gave centrifugal a special name to make it an easy to grasp concept, but at the end of the day, it is the same force we feel as we are resisting the change in speed (acceleration) of a car in a straight line. I dont know why they neglected to name that feel too??
Wait...what about "g forces"?
Remember Lu,
Stick to the logic chain. Stop at the step that does not ring true and discuss that step BEFORE going on to the next.
Good luck.
I must drink heavily now.....
