Centrifugal Force: Is it Radially Outwards?

[Cantab Morgan]

Newtons third law does have the forces occurring in pairs but it also requires the forces to be :
1.of the same type
2.equal in size
3.opposite in direction
The above three criteria are not met when considering the centripetal force on the stone and any pull on the string.

What does "of the same type" mean? I've never heard that before.

 

[D H]

What does "of the same type" mean? I've never heard that before.

A specific example: Some people say that the upward force exerted by a scale on an object atop the scale is a third law reaction to the gravitational force exerted by the Earth on the object. This is erroneous. For one thing, the object on the scale is undergoing uniform circular motion about the Earth's rotation axis (the Earth is rotating, after all). The net force on the object on the scale cannot be zero, and thus the upward force exerted by the scale is not equal but opposite to the gravitational force.

Even more basic, the two forces are not of the same type. The downward gravitational force results from gravity. The upward force by the scale is something else: It is a manifestation of the electrostatic force. This is not a third law reaction force. It is something else.

 

[BobbyBear]

You mean with your hand (holding the string) below the stone?

Not possible unless you're cheating.

Um, so how exactly does one cheat? I mean, when we see that happening, what's really happening? o:

 

[tiny-tim]

Um, so how exactly does one cheat? I mean, when we see that happening, what's really happening? o:

Dunno … computer graphics, maybe?

Have you ever seen it happening?​

 

[BobbyBear]

Dunno … computer graphics, maybe?

Have you ever seen it happening?​

Um, I thought it was commonplace to see it happening, swinging a lasso over your head, like the cowboys do . . . in fact, I'm going to try doing it right now:P:P

-goes to swing rope around head-

-back-

Uhuh, to be honest I can't do it very well, the rope looks to be pretty much horizontal, but I guess that's because I'm trying to swing it very fast :P

So it's impossible to do? I mean, I thought when you said it would be 'cheating', I thought you meant that it could be done but by doing something we were not considering in our model, some addtitional effect we weren't considering, like some sort of upward force our wrist is exerting upon the rope (which is not present in the rotating shaft that has a string with a mass tied to it). I'm just asking, when we swing a rope with an upward tilt (if it's possible, lol, now I'm not so sure), we must be exerting an upward impulse or something? I was just wondering how you'd explain it, and how you'd model it . . . as an upward force?

 

[tiny-tim]

See if you can find a video of it …

I don't think you'll find one of the rope staying above the level of the hand.

 

[Dadface]

As you whirl the rope faster and faster it approaches the horizontal but never quite reaches.The mass on the rope has two main forces on it mg, and the tension in the rope(T).Tsin theta supports the weight and T cos theta provides the centripetal force(theta =angle rope makes to the horizontal).We can write:
Tsin theta=mg...1
Tcos theta=mv^2/r...2
Divide equation 1 by equation 2 to get a value for tan theta and you will see that for theta to become zero and the rope to become horizontal v must become infinite which is impossible.