Centrifugal Force Question

[dansmithers]

I am in the planning stages of a plugin (my very first!) and I have question about the kinematics of bodies rotating about a point.

Knowing the angular velocity and the point of origin, how does one find the distance from origin to body?

[Mr Shifty]

You don't have enough information there, unless you know something about the forces involved. For instance, put a ball on the end of a string and you can spin it around at arbitrary angular velocity (limited by gravity and the tensile strength of the string.)

[lajoswinkler]

This is fairly simple and should help you.

http://www.calctool.org/CALC/phys/newtonian/centrifugal

[dansmithers]

You don't have enough information there, unless you know something about the forces involved. For instance, put a ball on the end of a string and you can spin it around at arbitrary angular velocity (limited by gravity and the tensile strength of the string.)

Ah-I thought that was all I needed. What would you need to calculate that?

EDIT: Thanks Lajo!

Edited October 1, 2014 by dansmithers

[Idobox]

There are several ways to get the distance.

The easier one is if you have the coordinates. You just do distance = sqrt ( (xo-xb)² + (yo-yb)² + (zo - zb)² ), o denoting origin and b body. If by origin you mean the origin of the coordinate system, then xo=yo=zo=0, but from the your question, I suppose you mean the centre of the planet.

On thing to pay attention to: if you want to compute centrifugal force, what you need is the distance to the axis of rotation. For example, on Earth, the axis goes from pole to pole, and you feel no centrifugal force from Earth rotation when standing on a pole, and more and more as you get closer to the equator.

The maths of how to get the force will depend on the coordinate system you use (I think KSP uses Cartesian, ie x y z, but I'm not sure), and the way you represent your rotation (typically angular velocity around the three axes).

[Dominatus]

Might be nitpicking here.. But I had thought centripetal force was the correct terminology, that centrifugal force was a myth? Am I wrong?

[lajoswinkler]

Might be nitpicking here.. But I had thought centripetal force was the correct terminology, that centrifugal force was a myth? Am I wrong?

No. It's a myth that it's a myth, and it's spread by teachers not competent enough to explain it correctly.

Centripetal force is the force pulling the body towards the center. It might be the electrical force of the molecules in the rope spinning a bucket or gravity holding the Moon, doesn't matter.

Centrifugal force is very real, but so called pseudoforce. It exists only in the rotated object's point of reference and is arises from the fact that there's a contiuous acceleration of the vector of the velocity.

Velocity is speed and direction. While the speed remains the same, the direction changes. It's a type of acceleration which isn't something we can normally understand, though, but it exists.

If there wasn't such thing as a cetrifugal force as the result of a vector acceleration (let's pretend for the sake of argument and ignore everything else), you'd only have centripetal force. The rope would pull in the bucket, the Moon would fall on Earth. In real world, the centripetal force and centrifugal (pseudo)force are equal and the bucket spins around your hand, and the Moon around Earth.

One of the notable pseudoforces is so called hydrophobic force in cells. It's a completely different thing from centrifugal, but it just shows there are such things as pseudoforces. Not apparent outside a specific point of reference.

[Mr Shifty]

Centripetal force is the force pulling the body towards the center. It might be the electrical force of the molecules in the rope spinning a bucket or gravity holding the Moon, doesn't matter.

Centrifugal force is very real, but so called pseudoforce. It exists only in the rotated object's point of reference and is arises from the fact that there's a contiuous acceleration of the vector of the velocity.

Centrifugal force arises in the rotating frame because inertia operates in the non-rotating frame. If no force is applied, the object travels in a straight line in the non-rotating frame; centripetal force is the force that curves the object's path. In the rotating frame, the centrifugal vector represents the contribution of inertia in the non-rotating frame. This is why gravity is fictitious in GR; objects travel along geodesics with no force applied, and gravity represents the contribution of inertia in curved space-time.

[Dominatus]

Ohhh, okay. Thanks, I may have misunderstood my physics teacher last year when we discussed this concept, or he was simply incorrect. Thank you for clearing this up!

[Linear]

Another way of thinking about it is a bucket full of water being swung around on string, centifugal force is the contact force created by the water pushing the bottom of the bucket. Centripetal force is the force required to create an inward-circle acceleration.

[WestAir]

Gravity is a fictitious force in GR?

[Mr Shifty]

Gravity is a fictitious force in GR?

http://en.wikipedia.org/wiki/Fictitious_force#Gravity_as_a_fictitious_force

[K^2]

Gravity is a fictitious force in GR?

Absolutely. The way you find a trajectory of an object in GR is by solving for zero acceleration. So effectively, you find a coordinate system in which object follows Newton's First Law, and then if you need the force of gravity, you simply find the fictitious force it'd take to drag the object along that trajectory with respect to your choice of coordinates. It's the same idea as with rotating frames of reference in classical mechanics.

So as an interesting side effect, in GR you don't need to treat gravity and rotation separately. You can just pack rotation into your metric, and any centrifugal/Coriolis effects will factor into gravity you get.

[WestAir]

That is the single most amazing thing I've read learned in a while.

[KerikBalm]

This seems appropriate here:

http://xkcd.com/123/

The mouse-over caption: "You spin me right round, baby, right round, in a manner depriving me of an inertial reference frame."

[Tommygun]

I thought the confusion is between Centrifical vs Centrifugal Force.

Centrifical is a corruption or misspelling of the word centrifugal.

I think that may have lead to more confusion that centrifugal force doesn't exist, at least in part?

[lajoswinkler]

I thought the confusion is between Centrifical vs Centrifugal Force.

Centrifical is a corruption or misspelling of the word centrifugal.

I think that may have lead to more confusion that centrifugal force doesn't exist, at least in part?

Who the hell uses the word "centrifical"?

[Tommygun]

Who the hell uses the word "centrifical"?

From what I have read it's a weird American corruption of the word that started in the 19th century and just kept on going.

It's not seen much outside of the US.

[WestAir]

I've lived from the West Coast to the East Coast, and one of my jobs keeps me in earshot of something like 65,000 (new) Americans every day. Never have I heard the word "centrifical" force.

[Mr Shifty]

From what I have read it's a weird American corruption of the word that started in the 19th century and just kept on going.

It's not seen much outside of the US.

Uh, it's not seen much inside the US either, at least among people who know how to spell. The confusion about centrifugal force stems entirely from first-year physics teachers. I mean, centrifugal force obviously exists; you can experience it easily simply by spinning around and feeling the force that tends to lift your arms away from your sides. You could put a strain gauge or even a spring scale on a centrifuge and record how the force imparted by a mass resting on the scale increases with increasing angular velocity. What causes the spring to compress in the non-rotating frame is the reaction force that the mass exerts on the scale when then scale pushes on the mass to curve its path. And this very real reaction force has precisely the same magnitude as the centrifugal force in the rotating frame. So, in this case, the 'centrifugal' force is utterly real. Where it breaks down is when an object is not acting under centripetal acceleration and thus is not subject to reaction forces; it will appear to accelerate radially from within the rotating frame, but is either stationary or moving inertially in the non-rotating frame.

Edited October 3, 2014 by Mr Shifty

[lajoswinkler]

From what I have read it's a weird American corruption of the word that started in the 19th century and just kept on going.

It's not seen much outside of the US.

It sounds like it should go in the same bin as nucular.

[EdFred]

I always thought that was called inertia.

[Tommygun]

I've lived from the West Coast to the East Coast, and one of my jobs keeps me in earshot of something like 65,000 (new) Americans every day. Never have I heard the word "centrifical" force.

Well it's not like the word comes up in everyday conversations and people who work in science or engineering should know better.

Lajoswinkler, in fairness that "nucular" thing is a regional Southern accent. It's like telling some British people they are saying aluminum or scheduling wrong. I see it as nether right or wrong, it's just the way some parts of the world pronounce things.

Edited October 4, 2014 by Tommygun

[lajoswinkler]

Well it's not like the word comes up in everyday conversations and people who work in science or engineering should know better.

Lajoswinkler, in fairness that "nucular" thing is a regional Southern accent. It's like telling some British people they are saying aluminum or scheduling wrong. I see it as nether right or wrong, it's just the way some parts of the world pronounce things.

That's not an accent nor dialect (I think you were talking about a dialect). Nuclear is a new, modern word. It entered the common language in the era of atomic weapons, so it can't be called a dialect; it's too young.

Nuclear comes from nucleus. Nucular doesn't come from anything. It's a bastardization of a word. I wouldn't patronize people from southern USA states by saying that's how all of them pronounce it. It's kind of insulting.