Centrifugal force?

[Dr Jekyll]

I appreciate the argument that it doesn't exit, newton's first law and all that. But it's a useful concept all the same. My view is that there is certainly such a thing as centrifugal force. It isn't technically a force, but we all know what we mean.

[Silent1]

yeah, you mean centripetal force.

[Dr Jekyll]

No I don't. If I spin a bucket of water around my head it's centripetal force keeping the water in the bucket. But if the base of the bucket gives way and water goes all over everywhere it isn't a force at all that distributes the water, it's just carrying on with the same velocity in the absence of a force. But it's useful to have a name for it and centrifugal force is understood even if it annoys pedants.

Edited by Dr Jekyll on Wednesday 9th April 15:29

[Engineer1]

Centrifugal force doesn't exist, Centripetal force does if you cut the string on the bucket it would fly off at a tangent to the curve it was describing.

[Dr Jekyll]

Engineer1 said:

Centrifugal force doesn't exist, Centripetal force does if you cut the string on the bucket it would fly off at a tangent to the curve it was describing.

Whatever causes it to fly off is not a force, it's flying off because it is no longer being acted on by centripetal force. It's useful to have way to describe the tendency of such buckets to fly off in these situations, and everyone knows what is meant by 'centrifugal force', so let's call it centrifugal force.

You may as well object to references to 'Sea Horses' because they aren't really Horses.

[geeks]

Engineer1 said:

Centrifugal force doesn't exist, Centripetal force does if you cut the string on the bucket it would fly off at a tangent to the curve it was describing.

The existence of the centrifuge would seem to disprove your assertion

[Silent1]

geeks said:

Engineer1 said:

Centrifugal force doesn't exist, Centripetal force does if you cut the string on the bucket it would fly off at a tangent to the curve it was describing.

The existence of the centrifuge would seem to disprove your assertion

no it doesn't the force at work in a centrifuge is centripetal force.
The Phsyics of a Centrifuge

[tank slapper]

Whether centrifugal force exists or not depends on your frame of reference. If you are looking as an outside observer, then it doesn't exist. As pointed out above, the apparent force pulling the water outwards is just the reaction felt from the bucket preventing it continuing in a straight line. If you switch your frame of reference to the bucket (ie considering it to be stationary), then centrifugal force is necessary to explain the behaviour of the water.

Centrifugal force is an inertial force, which means it only exists as a result of motion and not through the interaction of objects. The wiki article on fictitious forces is quite good.

[CrutyRammers]

A useful real world concept IMO, if not actually technically correct. See also "hot air rises".

[grumbledoak]

It's just momentum, really, but it's a useful verbal shorthand as long as everyone does understand that.

[Simpo Two]

Silent1 said:

no it doesn't the force at work in a centrifuge is centripetal force.
The Phsyics of a Centrifuge

But then it would be a centripete...

And does every force not have an equal and opposite force?

[FurtiveFreddy]

Its an effect rather than a force, so if you want to use the word 'centrifugal', just replace 'force' with 'effect' and you won't be subject to pedantry.

[Shaoxter]

One of my faves from the geeks jokes thread:

[Gandahar]

If centrifuges with cheeks rippling on the test subjects gets more people interested in science then I am all for it.

Arguing the toss on semantics never got anyone excited in physics

[Tony2or4]

Simpo Two said:

does every force not have an equal and opposite force?

Indeed it does, but you have to be precise in interpreting what Newton's 3rd Law is trying to say: if body A exerts a force on body B, then body B simultaneously exerts an equal and opposite force on body A.

So in the case of the bucket, the bucket/string combo experiences a force towards the centre of the circle: that force comes from the bloke's arm; at the same time the bloke's arm experiences a force, provided by the string, in a direction outwards from the centre of the circle.

FurtiveFreddy said:

Its an effect rather than a force, so if you want to use the word 'centrifugal', just replace 'force' with 'effect' and you won't be subject to pedantry.

Well put, that man.

[Engineer1]

Tony2or4 said:

Simpo Two said:

does every force not have an equal and opposite force?

Indeed it does, but you have to be precise in interpreting what Newton's 3rd Law is trying to say: if body A exerts a force on body B, then body B simultaneously exerts an equal and opposite force on body A.

So in the case of the bucket, the bucket/string combo experiences a force towards the centre of the circle: that force comes from the bloke's arm; at the same time the bloke's arm experiences a force, provided by the string, in a direction outwards from the centre of the circle.

but if the force was reactive if the string was to break the bucket would fly off along the radius rather than tangentially

[tank slapper]

Engineer1 said:

but if the force was reactive if the string was to break the bucket would fly off along the radius rather than tangentially

No, it wouldn't. The centripetal force accelerates the bucket so that it travels in a circle. Because there is no movement inward or outward by the bucket and the radius is constant, there must be a reaction force balancing the centripetal force. If the string is cut then the acceleration stops instantly and the bucket continues in the direction it is already traveling, which is a straight line that is a tangent to the rotation. It can't suddenly travel straight along the radius because it is never moving along the radius anyway. To do so would require an immediate 90 degree turn.

[Simpo Two]

I smell it's something to do with 'angular velocity', and that the difference bewteen that and the other kind of velocity is the force. Or summink.

Perhaps if I'd been any good at maths once all the numbers turned into letters, I might be famous by now

[Tony2or4]

Simpo Two said:

Perhaps if I'd been any good at maths once all the numbers turned into letters, I might be famous by now

No you wouldn't. Maths was the only thing I was any good at and I'm still not famous.

ETA My being not famous is illustrated by the fact that when I googled my own name, the only thing that came up was a reference to me retiring in the school's online magazine.

Edited by Tony2or4 on Friday 11th April 18:34

[Tony2or4]

tank slapper said:

Engineer1 said:

but if the force was reactive if the string was to break the bucket would fly off along the radius rather than tangentially

No, it wouldn't. The centripetal force accelerates the bucket so that it travels in a circle. Because there is no movement inward or outward by the bucket and the radius is constant, there must be a reaction force balancing the centripetal force. If the string is cut then the acceleration stops instantly and the bucket continues in the direction it is already traveling, which is a straight line that is a tangent to the rotation. It can't suddenly travel straight along the radius because it is never moving along the radius anyway. To do so would require an immediate 90 degree turn.

Most of what you say here is spot on, TS, apart from the bit about there being a reaction force balancing the centripetal force.

Because it's moving in a circle - ie moving with non-constant velocity (even if the speed is constant) - the bucket is undergoing an acceleration, and the only force needed for that to happen is the centripetal force (in the form of tension in the rope).

The reaction force (as in Newton's 3rd law) is the force experienced by the bloke in his arm.