Is the concept of reactive centrifugal force valid?

[harrylin]

So only one of the two objects in this example exerts a centrifugal reactive force? So what is the reaction force for the centripetal force supplied TO the space station (ie the one that causes its centre of mass to accelerate toward the centre of mass of the station/passenger system)? What is it exerted on and what direction is it?

AM

First (as I originally understood your question), ignoring the little imbalance caused by the astronaut, there is no centripetal force on the space station. When it rotates, its molecular adhesion forces allow the space station to stay in one piece and to accelerate the astronaut. In order to maintain the centripetal acceleration of the astronaut by the space station, the floor of the space station must exert a centripetal force on the astronaut. And of course, the astronaut exerts a centrifugal counter force on the floor of the space station.

But as you correctly noticed, it is a quite a bit trickier when we look, not at the motion of the floor, but at the motion of the centre of mass of the space station. Good one.

As you indicated, the centrifugal force that is exerted on the floor will cause a slight imbalance that results in a little circular motion of the centre of mass of the space station. It can only be the centrifugal force that is exerted on one point on the floor that takes care of the centripetal acceleration of the centre of mass of the space station (in the same direction: for example the centre of mass of the space station can be slightly to the left of the centre of the system while the astronaut is exerting centrifugal pressure on the floor on the right).

 

[rcgldr]

Regarding the Wikipedia article which spawned this whole thread. In the first sentence of the first paragraph it says "This reactive force is directed away from the center of rotation".

In both Wiki articles, that statement is made in reference to a mass being accelerated by direct contact (implied) with some other object, not by a field (electrical, gravitational, magnetic):

This centripetal acceleration is caused by a force exerted on the mass by some other object. In accordance with Newton's third law of motion, the mass exerts an equal and opposite force on the object.

So the statements as written are OK, but it should be noted that when centripetal acceleration is due to a field (and not direct contact with an object), then both forces are usually centripetal. This has already been mentioned in the wiki discussion page for centrifugal force.

 

[sankalpmittal]

In both Wiki articles, that statement is made in reference to a mass being accelerated by direct contact (implied) with some other object, not by a field (electrical, gravitational, magnetic):

This centripetal acceleration is caused by a force exerted on the mass by some other object. In accordance with Newton's third law of motion, the mass exerts an equal and opposite force on the object.

So the statements as written are OK, but it should be noted that when centripetal acceleration is due to a field (and not direct contact with an object), then both forces are usually centripetal. This has already been mentioned in the wiki discussion page for centrifugal force.

If you whirl a stone tied to a string , then the restoring force of string which acts towards the centre and acts on the stone is centripetal force.

But since you know Newton's third law , there must be opposite reaction too existing in pairs. This opposite reaction is mentioned as Reactive centrifugal force. It acts on your hand equal and opposite to the direction of centripetal force.
However my text refers that opposite reaction must not be confused with the name of centrifugal force.

In rotating frame of reference , there is no centripetal force. The only force which acts is inertial centrifugal force, unlike whirling frame of reference.
Your inertia makes you stable there.

 

[D H]

In rotating frame of reference , there is no centripetal force. The only force which acts is inertial centrifugal force, unlike whirling frame of reference.

The centripetal force is a real force. It is present in all frames of reference. The centrifugal force is only present in rotating frames.

Your inertia makes you stable there.

No, it doesn't.

Care is needed here with regard to which rotating frame is being referenced. Presumably you are talking about a frame in which the moving object (moving as viewed from the point of view of an inertial frame) appears to be at rest.

What does Newton's first law tell you about the net force on an object that is at rest?