How about a little physics talk?

Bob_in_OKC

Addicted Member
I sometimes hear mention in this forum of centripetal force, including one mention in yet another of many RCM threads. So, anyone up for a detailed discussion, preferably with some examples for the layman? Where and how are centripetal forces at work, in practical terms? Anyone ready to take a shot at it?

To all who are reading this original post - Feel free to change the subject to anything related to the physics or geometry of turntables. We left centripetal force a while ago.
 
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I sometimes hear mention in this forum of centripetal force, including one mention in yet another of many RCM threads. So, anyone up for a detailed discussion, preferably with some examples for the layman? Where and how are centripetal forces at work, in practical terms? Anyone ready to take a shot at it?

One example of centripetal force: If one is driving fast in a curve and one is then pressed against the door, the force that the door will have on you is a centripetal force.
 
gusten's example is a good one. Centripeital force is the (measurable) force that the door exerts against the body of the person in the car; to counteract the centrifugal effect of the object (car and everything in it) changing direction (centrifugal effect is not measurable). You can only measure the force holding the "body" back.
 
'Ever ridden in the "Rotor" at an amusement park?

margate-dreamland-26.jpg
 
Is the absence of force, bodies go at a constant speed in a constant direction. Centripedal force is what the door exerts on you to make you go in a curved direction.

Centrifugal force is a "ficticious" force because you are in a non-inertial frame of reference.

I think I got that right, Classical mechanics was about forty years ago.

My favorite xkcd comic...

http://xkcd.com/123/
 
I don't know if I'm ready to accept that the car door is the source of centripetal force. The car is riding along with the people in it. The centripetal force seems to be coming from the road.

That aside, my intent was not to ask for an example of centripetal force on its own. I'm interested in those who can defend the contention that centripetal force is part of how a turntable works.
 
I don't know if I'm ready to accept that the car door is the source of centripetal force. The car is riding along with the people in it. The centripetal force seems to be coming from the road.

That aside, my intent was not to ask for an example of centripetal force on its own. I'm interested in those who can defend the contention that centripetal force is part of how a turntable works.

The centripetal force on the car is coming from the friction force tires/road. Not on me if sit in the car, because it´s my mass that is accelerated in that case.

In what way do You mean?
 
Just as everything in the car when it turns, and everything in the amusement park rotor is accelerated toward the outermost point(s) of the rotating object, anything riding on a record, riding on a rotating turntable will be accelerated toward the outermost point(s). Centripetal force is the force that resists motion associated with the acceleration.
 
Just as everything in the car when it turns, and everything in the amusement park rotor is accelerated toward the outermost point(s) of the rotating object, anything riding on a record, riding on a rotating turntable will be accelerated toward the outermost point(s). Centripetal force is the force that resists motion associated with the acceleration.

Yes the dust on the record will be. :D
 
The turntable is very different from the amusement park rotor. In the rotor, the people are traveling in a circle but would sail off in a straight line if the centripetal force were removed. On a turntable, the object riding on the record is virtually stationary, held in place by two groove walls and gravity.
 
Any force that keeps you moving in a circle.

Friction: roda on tires while a car turns a corner. A block placed on a turntable, and the platter rotated.
Tension: twirl a ball attached to a string.
Gravity: orbits
Normal (contact): what has been mentioned- car door (without its presence, you continue in a straight line down the road and out the door opening). Also, the Gravitron ride. Blink the cylinder out of existence and the riders no longer will move in a circle.


… just to name a few.

Someone said, "Centripetal force is the force that resists motion associated with the acceleration.". This is not true. Centripetal forces introduce an acceleration because even though your speed may be constant, your velocity (speed + direction) changes.
 
The turntable is very different from the amusement park rotor. In the rotor, the people are traveling in a circle but would sail off in a straight line if the centripetal force were removed. On a turntable, the object riding on the record is virtually stationary, held in place by two groove walls and gravity.

If You mean the needle there is no centripetal force at all acting on it. There is no acceleration on it therefor no force.
 
It all has to do with vectors. A car is complicated because there are tires and suspension, friction with the seats, seat belts, frame against body, etc working to break the body's directional velocity in a straight direction.

The cartridge wants to stay straight in the vector direction it is moving. The connection to an anchored tone arm works with anti skate and the record groove pressure on the stylus to accomplish desired tracking. Table suspension and the like just takes out variables.

When you accelerate the mass of the cartridge, it wants to stay in that direction. The tone arm, counterweight, and friction against the groove fight that desire.
 
To answer the turntable question, the outer groove provides a contact force that pushes the needle continuously to the center of the spindle.
 
To answer the turntable question, the outer groove provides a contact force that pushes the needle continuously to the center of the spindle.

That is not correct, for doing so the needle must follow the record, that is rest on the record.
 
OK- after thinking more on it, your question is a good one and ponder-worthy. It is not an easy question to answer!

Superficially, there must be a net force acting toward the center of rotation if the tonearm is to move in that direction. But that alone does not qualify it as centripetal. I shall give this more thought from other inertial reference frames.
 
OK- after thinking more on it, your question is a good one and ponder-worthy. It is not an easy question to answer!

Superficially, there must be a net force acting toward the center of rotation if the tonearm is to move in that direction. But that alone does not qualify it as centripetal. I shall give this more thought from other inertial reference frames.

Does anti-skate force not come into play?

Along with the central anchor of the tone arm.
 
There can only be a centripetal force on objects that are stationary on the record, following the record around.
 
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