What exactly is the reactive centrifugal force (split)

Andrew Mason

Well, I was assuming you were talking about a stress that is the result of two opposing forces that act on the same body in a way that tends to shear - make parts of the body that are close together move in opposite directions. Not sure how that applies here so if I am missing something, feel free to enlighten me.

No mention of shear stress in any of my physics texts. Sounds like engineering.

AM

A.T.

I really think this is overkill.

If you want a scenario where the centrifugal reaction causes centrifugal acceleration in the rotating frame, then take a turntable where a mass on rollers pushes a sliding mass outwards .

If you want a scenario where the centrifugal reaction causes centrifugal acceleration in the inertial frame, then take the rocket on a circular path.

Of course, none of this is really important, because Newton's 3rd applies regardless of acceleration.

DaleSpam

You are correct, the forces in question are the forces from the bolts and the force from the astronaut's feet. Those forces are in opposite directions and cause the shear stress in the floor. As you correctly indicate, parts of the floor which are close together tend to move in opposite directions, the part closer to the feet tends to move out while the part closer to the bolts tends to move in. This is shear stress.

When the bolts are cut the shear stress in the floor next to the bolts is relieved, but not in the rest of the floor. A shear wave ripples through the material at the speed of sound in the material relieving the shear stress as it goes. Until the shear wave reaches it, the material is in the same state of stress as it was before the bolts were cut.

I can go into more detail if you like, but A.T. is right, none of it is important. The centrifugal reaction force depends on the point of application of Newton's 3rd law, not any acceleration. You can argue with me and A.T. if you like, but even if you do convince us that it is bad terminology, that wouldn't make the bad terminology go away.

Andrew Mason

But that is a perfect example of the fictitious centrifugal force, not the reaction to the centripetal force on the mass. This confusion is the real problem.

That is an interesting scenario but I don't think that the force on the rocket qualfies as a centripetal force.

Centripetal force on a body defined as is dp/dt of the body where dL/dt of the body = 0. But if dL/dt for the rocket + fuel = 0 the rocket would have to constantly be increasing its rotational speed because it is constantly losing mass.

This is correct. And in an inertial frame where dL/dt = 0, all dp/dt is centripetal.

AM

A.T.

Yes, confusing fictitious and real forces is your real problem. In my scenario the sliding block can be very light and have large friction, which by far outweighs the sliding block's fictitious centrifugal force. What moves the sliding block outwards is the real centrifugal force from the inner heavier low friction block.
Based on your criteria most names would apply only to the simple idealized cases, that are found at the begin of a book chapter, where the name is first introduced. Most people who have some abstraction skill prefer more generally applicable names.
That is just the simplest possible case, not the general case.
Fine, if it makes you happy let the speed increase. Or replace the rocket with a plane, flying in a circle. It accelerates air centrifugally, to have a centripetal force.

DaleSpam

Or you can simply reduce the thrust in proportion to the reduction of mass. I do like the airplane idea also.

rcgldr

In the case of a rocket or aircraft, aren't both the centripetal and centrifugal forces "reaction" forces? For the aircraft, the centripetal force is the reaction (or coexistance) to the air being accelerated outwards, and the centrifugal force is the reaction (or coexistance) to the aircraft being accelerated inwards.

A.T.

The "action/reaction" label is meaningless. It just indicates that it is a 3rd law force pair.

Andrew Mason

What moves the block outward is inertia. The inability of the friction force between the turntable and the block to provide the centripetal acceleration required to keep both blocks on the turntable, results in the blocks not accelerating sufficiently to stay on the turntable. There is no outward force.

It is the only case in which the force is centripetal - that is constantly directed toward the same inertial point. If dL/dt ≠ 0 the force is not centripetal - it has centripetal and tangential components so it is not pointing toward the same central point.

AM

A.T.

That was my original idea, but Andrew wants L = const. So let's speed up to compensate for mass reduction.

The same in space would be a ship with a solar sail, doing small circles at constant distance to the sun. The radial lift component cancels gravity. The other component provides centripetal acceleration. The reaction to the centripetal force accelerates particles centrifugally.

A.T.

That's mumbo jumbo. You have to use forces acting on the outer block to explain the outer block's outwards acceleration in the rotating frame. There is a fictions centrifugal force acting on the outer block in the rotating frame, but it is not sufficient to overcome friction. Only because of the real centrifugal force from the inner block the outer block accelerates outwards in the rotating frame.
There are even two in the rotating frame.
So there is a centripetal component, and a centrifugal reaction component.

rcgldr

I assume the sail reflects the particles back towards the sun, so the sail exerts a centripetal force (and acceleration) onto the particles (in addition to gravity) and the particles exert a reactive centrifugal force onto the sail.

A.T.

The ship makes small circles in a plane that is perpendicular to the solar wind and gravity. The sail is at an 45° angle outwards to the solar wind and gravity. The particles are diverted by 90° and fly off centrifugally within the plane of the circular path.

DaleSpam

For a rocket and its exhaust dL/dt=0 regardless of how fast or slow or in which direction you point the exhaust. It is a great example for that reason.

rcgldr

OK, but even if the particles are diverted 90°, the direction of acceleration of those particles includes a component towards the sun (radial outwards velocity is reduced to zero), and a tangental component (at that instant in time).

A.T.

If I understand him correctly he wants L = const for the body on which the centripetal force is exerted, which is the rocket and the remaining fuel. But the example can be adjusted to work either way.

A.T.

That is not relevant of the circular motion. In the plane of the circle there is a centripetal force and a centrifugal reaction, which accelerates the particles outwards.