Centripetal force vs centrifugal force

[Steve]

Many years of reading epic ski and it became quite clear to me that centrifugal force is not real and that the term centripetal force should be used (so it's been said many times.)

I can feel this when it comes to skiing as the snow is pushing back on my skis. So the inward force makes sense.

However if you take a ball on a string and spin it around there is no push back, it is entirely a momentum caused outward force. In fact cut the string and it will fly outward.

I'd love to hear some hopefully simple explanations of the difference between these two terms and why it is so oft criticized to use the term centrifugal force.

 

[RuleMiHa]

A lead weight swinging on a string around in a circle has it's centripetal force (centrally directed force) applied by the string, and cutting the string will result in the lead weight flying off at a right angle to the inward force vector, not actually outward.

The force the snow exerts on skis acts in the same way the string does. It directs inward forces on the skier which result in a turn.

I have a difficult time understanding what centrifugal force is supposed to be, because it's really not a force in the same way. People seem to use it to describe the sensation one feels when being acted on by centripetal force.

For instance, in a turn the force is most definitely centrally directed but we feel as if we are being pushed outward. That feeling is not a force, but it is a real feeling.

So in common usage I would say centripetal forces are the actual forces that act on us in a turn, whereas centrifugal (forces) are the sensations we perceive while being acted on by centripetal forces but are not real forces? Does that make sense?

Engineers, am I right/wrong? I really hated physics. A lot. Really.

 

[cantunamunch]

I'd love to hear some hopefully simple explanations of the difference between these two terms and why it is so oft criticized to use the term centrifugal force.

Centrifugal is perfectly appropriate for use when discussing forces and acceleration within a rotating frame of reference relative to an inertial frame of reference.

Centripetal is perfectly appropriate for use when discussing forces and acceleration within an inertial frame of reference.

https://en.wikipedia.org/wiki/Rotat...ation_between_accelerations_in_the_two_frames

For instance, in a turn the force is most definitely centrally directed but we feel as if we are being pushed outward. That feeling is not a force, but it is a real feeling.

The term you are looking for is 'fictitious force'. https://en.wikipedia.org/wiki/Fictitious_force The word 'fictitious' here is a technical physics term and is NOT synonymous with 'not real'. People who attempt to simplify this concept are the ones creating the problem.

So in common usage I would say centripetal forces are the actual forces that act on us in a turn, whereas centrifugal (forces) are the sensations we perceive while being acted on by centripetal forces but are not real forces? Does that make sense?

You're trying to dip your toe into the inertial reference frame while being inside a rotating one. Your definition is usable IF and only if you are very very pedantic about who what and where is turning - but it doesn't simplify anything.

Engineers, am I right/wrong? I really hated physics. A lot. Really.

Except for the 'reality' part, you're not blatantly wrong It's just that the form in which you phrase it doesn't keep the two frames distinct and therefore mistakes WILL occur.

 

[PeteW]

It is similar to standing upright in accelerating vehicle (train or bus). The force which accelerates the vehicle is directed forward, but you feel as being pushed backward.

 

[François Pugh]

Centripital is an adjective meaning towards the centre, e.g. towards the centre of the circle matching the arc your skis are making. It can be used to describe the force acting on your skis at the ski/snow interface in any reference frame.

Centrifugal is an adjective meaning fleeing or away from the centre, e.g. away from the centre of the circle matching the arc your skis are making. It can be used to describe the force acting on the snow at the ski/snow interface in any reference frame.

Usually, we choose the biggest thing around as our frame of reference, and consider it stationary, e.g the ground. All of Newton's laws work fine (so long as we are not moving near the speed of light). They work for forces applied at interfaces like the ski/snow interface and for body forces like gravity deemed to exist from studying falling objects over centuries.

Sometimes we can simplify the math by choosing a different frame of reference, e.g. a train traveling at constant speed down a track. All of Newton's laws work fine (so long as we are not moving near the speed of light).

Now if we choose the train as our frame of reference and the conductor is slamming on the brakes, and we examine the situation armed with Newton's laws, we observe that objects in the train are being pulled to the front of the train by a body force, just as they are being pulled down by gravity. Using this accelerated (speed changing or direction changing or both changing wrt to our usual earth-based frame of reference) frame of reference a force related to the acceleration of the frame of reference exists and Newton's laws work out.

Now if we choose the train as our frame of reference and it is traveling at constant speed around a curve, and we examine the situation armed with Newton's laws, we observe that objects in the train are being pulled to the outside of the curve by a body force, just as they are being pulled down by gravity. Using this accelerated (speed changing or direction changing or both changing wrt to our usual earth-based frame of reference) frame of reference a force related to the acceleration of the frame of reference exists and Newton's laws work out.

In skiing physics we usually stick to two frames of reference: fixed to the ground is one of them; accelerating in a curve with the skier is the other one, sometimes used because it matches our experience. Do not try to use both frames of reference at the same time!

 

[Ski&ride]

I have a difficult time understanding what centrifugal force is supposed to be, because it's really not a force in the same way. People seem to use it to describe the sensation one feels when being acted on by centripetal force.

So in common usage I would say centripetal forces are the actual forces that act on us in a turn, whereas centrifugal (forces) are the sensations we perceive
have a difficult time understanding what centrifugal force is supposed to be, because it's really not a force in the same way. People seem to use it to describe the sensation one feels when being acted on by centripetal force.

You understand it right...up till the point of “when being acted on by centripetal force”!

The sensation is actually Newton’s 3rd law “object will continue moving (in a straight line)...”

It is always there whether they’re being pushed back by centripetal force or not.

If you miss you turn, centrifugal “force” takes you into the trees. If you made the turn, you’ve utilized centripetal force from the snow correctly.

 

[mdf]

(from XKCD)

 

[geepers]

All of Newton's laws work fine (so long as we are not moving near the speed of light).

If you are going to mention speed of light why not also include the realm of the very small where Newtonian physics also breaks down.

And we don't have to be going anywhere near the speed of light. The GPS satellites meander along at a mere 14,000 kph yet their clocks must be adjusted for both special (for the speed differential) and general relativity (for the gravitational field differential) otherwise positional errors would accumulate at the rate of about 10 km per day.

Fortunately for us skiers we need consider neither relativity nor quantum mechanics.

Since my post pinged by the moderator (valid point), I'll rephrase it more politely: Since skier speeds and sizes are well within the domain of newtonian physics could we kindly refrain from invoking relativity, quantum mechanics and the Higgs-Boson in these discussions.

 

[RuleMiHa]

Centrifugal is perfectly appropriate for use when discussing forces and acceleration within a rotating frame of reference relative to an inertial frame of reference.


You're trying to dip your toe into the inertial reference frame while being inside a rotating one. Your definition is usable IF and only if you are very very pedantic about who what and where is turning - but it doesn't simplify anything .

.

In skiing physics we usually stick to two frames of reference: fixed to the ground is one of them; accelerating in a curve with the skier is the other one, sometimes used because it matches our experience. Do not try to use both frames of reference at the same time!

Horrifyingly enough, I actually understand the frame of reference stuff but am confounded by the need to keep it separate. It's all happening to me at the same time, right?

Would it be more correct to say that someone watching me ski (inertial FOR) sees centripetal forces acting on me, but I as the skier (rotational FOR) feel centrifugal forces?

 

[skix]

(from XKCD)

Very nice. Of course hovering over an XKCD comic always brings a bonus:

 

[coops]

I reckon most of the confusion comes down to one person in the discussion speaking (or writing) colloquially, while the other is using the more strictly defined & 'more correct' language of physics.

If you ask me my weight I'll happily reply that I weigh 83 kg.

This is, 'of course' , wrong.

I do NOT weigh 83 kg... my mass is 83 kg, whereas my weight is about 813 N (when not moving around on the planet Earth ;-) ... in other words when my mass is accelerated upon by Earth's gravitational field ). If I jump up and down on some bathroom scales I can see my weight changing due to acceleration effects I am exerting on my body.

Of course... if you ask a Nobel prize winning Physicist in a bar how much he weighs... they're not going to answer in the proper scientific units because they'll sound like a weirdo

So... the stone whirling around in a circle on the end of a piece of string is continuously accelerating - and centripetal force along the string towards your finger (the centre of rotation) keeps it going round in that circle. Cut the string, and the object will move off at a tangent to the arc it was travelling on when you cut it as it no longer has the string to exert that centripetal force and provide the necessary acceleration...

which bings up confusion between speed and velocity and acceleration... again due to colloquial English vs Sciencey speak.

We 'normally' (colloquially) think of say a car accelerating when its speed is increasing... and kind of forget that what we are really dealing with is velocity and acceleration, and that velocity takes account of both direction and speed.
What this means is that any time your direction is changing your velocity changes... so anything ( a rock on the end of a piece of string being spun in a circle, a car going round a corner, a skier making as turn ) is accelerating (even though our speed may be constant.. our velocity isn't).

So a skier making a turn (or a driver taking a sharp left hand corner at speed, sitting in supportive Recaro seats) will feel that centripetal force being exerted on themselves - there is no centrifugal force to 'throw' you into the trees... there's just the risk of loss of centripetal accleration due to dodgy tyres or badly tuned ski edges and/or bad technique

Disclosure - I am not a Physicist.

ps having brought up mass and weight thanks to gravity... here's an 'amusing' misconception to wrap your head around...
"Einstein showed through the use of complex mathematics that gravity is not a force per se, but a consequence of the curvature of spacetime. Einstein discovered that massive objects bend the spacetime around it, and we perceive of this bending of spacetime as a force."
Say what now?
Scroll down to #8 of this link for the rest
https://futurism.com/moon-will-turned-giant-particle-detector

 

[mdf]

Cut the string, and the object will move off at a tangent to the arc it was travelling on when you cut it as it no longer has the string to exert that centripetal force and provide the necessary acceleration

Of course, in the rotating frame there is no arc, and the mass flies out straight (at first, then curving due to the Coriolus force).

 

[TheArchitect]

I just want to create consistent linked short-radius centripetal forces on steep terrain...

 

[James]

Of course, in the rotating frame there is no arc, and the mass flies out straight (at first, then curving due to the Coriolus force).

So in the rotating frame of reference- This is the same as "inertial"?
You would feel a cessastion of the force pushing from the right, then a force of friction from the front slowing you down? Assuming counterclockwise/left turning and flat hill.

 

[Rod9301]

Many years of reading epic ski and it became quite clear to me that centrifugal force is not real and that the term centripetal force should be used (so it's been said many times.)

I can feel this when it comes to skiing as the snow is pushing back on my skis. So the inward force makes sense.

However if you take a ball on a string and spin it around there is no push back, it is entirely a momentum caused outward force. In fact cut the string and it will fly outward.

I'd love to hear some hopefully simple explanations of the difference between these two terms and why it is so oft criticized to use the term centrifugal force.

Are you really learning physics in a ski forum?

 

[mdf]

Are you really learning physics in a ski forum?

When we talk physics here, I mostly participate for my own amusement.

You would feel a cessastion of the force pushing from the right, then a force of friction from the front slowing you down?

To be relevant for a moment, when you are pulling a high-force turn and your edges let go, it feels to you like you suddenly start going sideways. But to a stationary observer standing nearby, it is apparent that you are "really" still going mostly forward.

 

[Steve]

I just want to create consistent linked short-radius centripetal forces on steep terrain...

You will. You know what they feel like I'm sure, so you'll experience that feeling more and more.

 

[Steve]

Great posts everyone, thanks! I am no longer afraid to use both words.

 

[mdf]

Very nice. Of course hovering over an XKCD comic always brings a bonus:

View attachment 65697

If you are puzzzled, or just want to see all the fun ground out of a joke, you can go to the volunteer site "xkcd explained." I do like their tag line, though : "Explain xkcd: It's 'cause you're dumb."

https://www.explainxkcd.com/wiki/index.php/123:_Centrifugal_Force

 

[LiquidFeet]

Thank you, mdf, for that link. I now understand that my physics teachers lied to me, and why.
Just got a weee bit smarter.