# The Naked Scientists Forum

### Author Topic: Centrifugal or Centripetal?  (Read 2554 times)

#### DocSam

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##### Centrifugal or Centripetal?
« on: 08/12/2008 16:18:59 »
I remember learning at school (many years ago) that physically speaking, there was really no such thing as a centrifugal force, 'flying away from the centre'.
Puzzlingly, the notion of the centrifugal force is still perpetuated, especially in the media, and I have yet to hear anyone attempt publically to correct the disillusion.
Newton's Laws state that force equals mass times acceleration, and that every force must have an equal and opposite reaction, for the situation to be stable.  When a mass rotates about another - as in a planet around the sun, or a ball at the end of a piece of string - it is the 'tension' (i.e. force) between the two masses that supports circular rotating motion.  If that 'tension', whether gravitational or through a piece of string, breaks, then the masses will fly apart, but the rotating mass will not fly away perpendicularly, instead tangentially (at a tangent), indicating that there was no 'centrifugal force' (away from the centre), rather a 'centripetal' force (towards the centre), keeping the mass in stable circular motion.
Is this argument correct?  I have always believed so, but it has never been validated by anyone I know.

#### lightarrow

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##### Centrifugal or Centripetal?
« Reply #1 on: 08/12/2008 16:37:12 »
I remember learning at school (many years ago) that physically speaking, there was really no such thing as a centrifugal force, 'flying away from the centre'.
Puzzlingly, the notion of the centrifugal force is still perpetuated, especially in the media, and I have yet to hear anyone attempt publically to correct the disillusion.
Newton's Laws state that force equals mass times acceleration, and that every force must have an equal and opposite reaction, for the situation to be stable.  When a mass rotates about another - as in a planet around the sun, or a ball at the end of a piece of string - it is the 'tension' (i.e. force) between the two masses that supports circular rotating motion.  If that 'tension', whether gravitational or through a piece of string, breaks, then the masses will fly apart, but the rotating mass will not fly away perpendicularly, instead tangentially (at a tangent), indicating that there was no 'centrifugal force' (away from the centre), rather a 'centripetal' force (towards the centre), keeping the mass in stable circular motion.
Is this argument correct?  I have always believed so, but it has never been validated by anyone I know.
What you say is correct, but in the laboratory reference frame only.
In the rotating ref. frame the centrifugal force do exist (even if is called "fictitious" because is only due to the accelerating ref. frame).

#### lyner

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##### Centrifugal or Centripetal?
« Reply #2 on: 08/12/2008 17:21:04 »
A lot of anally retentive Physics teachers used to try to thrash the word 'centrifugal' out of us.
But they did have a point. *As lightarrow says, "When you 'let go' you don't fly away from the centre radially"  However, you do fly away, in as far as you get further away from the centre.
Newton III tells you that there must be a force in both directions. The outward force, however, acts on the thing at the centre and it disappears when the string breaks. In a centrifuge, the experience is certainly of a force acting (pushing you) 'against' the outside of the drum because you are actually in the rotating frame of reference.

It's just another example of over-teaching a point by people who don't fully understand what's going on. There was a time when you'd get murdered for referring to an electrical cell as a 'battery'.

« Last Edit: 08/12/2008 18:44:01 by sophiecentaur »

#### Bored chemist

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##### Centrifugal or Centripetal?
« Reply #3 on: 08/12/2008 18:14:45 »
"As lightarrow says, When you 'let go' you don't fly away from the centre radially. "
You do if you are stood on a  merry-go-round wearing roller skates.
(don't try this at home, but it shows that the reference frame matters.)

« Last Edit: 08/12/2008 18:44:44 by sophiecentaur »

#### lyner

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##### Centrifugal or Centripetal?
« Reply #4 on: 08/12/2008 18:39:30 »
If you drew a long, radial spoke out from the (rotating) roundabout and you let go, allowing yourself, initially, to move outwards along it, your trajectory (on ball bearings) would not be along that spoke; the spoke would keep moving around. It would 'leave you behind'. A trail of salt from your pocket would be curved - not radial.
« Last Edit: 08/12/2008 18:45:30 by sophiecentaur »

#### Bored chemist

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##### Centrifugal or Centripetal?
« Reply #5 on: 08/12/2008 19:16:57 »
In my (limited) experience roller skates go in straight lines.

#### lyner

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##### Centrifugal or Centripetal?
« Reply #6 on: 08/12/2008 20:06:34 »
Not when you fall off the edge!

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##### Centrifugal or Centripetal?
« Reply #6 on: 08/12/2008 20:06:34 »