## Main Question or Discussion Point

If a string is attached to a weight and swung around overhead, is the force that creates tension on the string centripetal, centrifugal, or both?

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DaveC426913
Gold Member
What do you think?

If a string is attached to a weight and swung around overhead, is the force that creates tension on the string centripetal, centrifugal, or both?
Centripetal. Centrifugal force can only be measured in a rotating frame of reference.

Pete

I think centripetal would always exist in the string. But, centrifugal (reactive) force would depend on the rigidity of the string.

Doc Al
Mentor
I think centripetal would always exist in the string. But, centrifugal (reactive) force would depend on the rigidity of the string.
Still insist on using that archaic meaning of centrifugal force as the "reaction" force to centripetal force? If so, then it's simple: The string pulls the object (centripetal) and the object pulls the string ("centrifugal"). Can't have one without the other.

One more time though: This is an old-fashioned usage of the term centrifugal. The modern usage is as a "fictitious" force that appears when viewing things from a rotating frame of reference.

Still insist on using that archaic meaning of centrifugal force as the "reaction" force to centripetal force? If so, then it's simple: The string pulls the object (centripetal) and the object pulls the string ("centrifugal"). Can't have one without the other.

One more time though: This is an old-fashioned usage of the term centrifugal. The modern usage is as a "fictitious" force that appears when viewing things from a rotating frame of reference.
Just to add to what Doc Al said, the concept of "force" is actually a very slippery one. Technically, a force is the rate of change of momentum away from what the natural trajectory would have been. In other words, it doesn't really make sense to talk about a force without fixing trajectories first. Normally, in an inertia frame, things move in straight lines (ignoring gravity). So if something moves in a circle, there must be a force to make it do so --- we call that "centripetal". However, if we're in a rotating frame, things tend to move away from the centre, so if something doesn't then you need a force --- we call that one "centrifugal".

Still insist on using that archaic meaning of centrifugal force as the "reaction" force to centripetal force? If so, then it's simple: The string pulls the object (centripetal) and the object pulls the string ("centrifugal"). Can't have one without the other.
archaic? Was it true? For the example which nuby explain,
a string is attached to a weight and swung around overhead
What are the action-reaction forces? Say the one is centripetal force, what is the other?

Doc Al
Mentor
The "action/reaction" pair is: String pulls on object & object pulls on string. When considering the motion of the object, we call the first force the centripetal force. The second force has no particular name. (Some folks call that second force the "centrifugal" force--but that's nonstandard usage. Old-fashioned and out of date!)

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DaveC426913
Gold Member
The "action/reaction" pair is: String pulls on object & object pulls on string. When considering the motion of the string, we call the first force the centripetal force. The second force has no particular name. (Some folks call that second force the "centrifugal" force--but that's nonstandard usage. Old-fashioned and out of date!)
I thought the second one was inertia - the object's tendency to continue moving in a straight line.

Doc Al
Mentor
Inertia is not a force.

Ok. centripetal-centrifugal forces pair never action-reaction forces.

Other hand, for this example, I suppose the pair of centripetal force is a force like so; say we pull a body by means of a string in space. There is no other force(friction, weight) then we apply. But anyway string will have has a tension. If this true, this is inertia force. But it is a pair-less fictitious force. Then how will we call it? :-/

Net force?

Hootenanny
Staff Emeritus
Gold Member
The string "pulls" on the body and the body "pulls" on the string, this forms a force-reaction pair. Equally, we "pull" on the string and the string "pulls" on us, again a force-reaction pair.

Doc Al
Mentor
Other hand, for this example, I suppose the pair of centripetal force is a force like so; say we pull a body by means of a string in space. There is no other force(friction, weight) then we apply. But anyway string will have has a tension. If this true, this is inertia force. But it is a pair-less fictitious force. Then how will we call it? :-/
As Hootenanny already explained, the string pulls on the object and the object pulls on the string. Two very real forces--nothing "fictitious" here.

Is it possible centrifugal force doesn't exist in the string at all? If you take the ball way and spin the string around by itself, the string would probably just get tangled around the axis. Would there be any centrifugal force in this example?

Maybe the string only transfers force to the weight? i.e. Weight pulls on spinning axis through string, and the rigidity of the string controls the rate force is transfered.

Doc Al
Mentor
Is it possible centrifugal force doesn't exist in the string at all?
According to your nonstandard definition of "centrifugal" force: NO. The string can't pull without being pulled.