Weight transfer?

[dipshit]

No, they're not. The CoG is the point at which the entire mass of the system can be considered to be concentrated.

That is the COM. The COG can move when other forces are applied. They are two different things.

They may be the same when static sitting on the lab table... but out in the dynamic real world they can be different.

[Mikkel]

Science defies common sense.

There's a lot of common sense here.

If you can not understand, that an object's COM and COG can only be located at the same point in space if the entirety of the object is positioned in a homogenous gravitational field, then you shouldn't pretend to be arguing anything related to physics. Just like well-meaning believers should not be allowed to teach intelligent design or other faith-based sentiments in a science class.

Shut up and go ride your motorcycle. Understanding the underlying mechanics is not going to make you a better rider. And a discussion of the underlying mechanics is only interesting provided that it is based on actual insight into the subject.

Comments like:

That is the COM. The COG can move when other forces are applied. They are two different things.

They may be the same when static sitting on the lab table... but out in the dynamic real world they can be different.

Are as scientific as "everything in this world is fire". It's just hyperbolic bullshit with no foundation in real insight or understanding. If believing otherwise makes you happy, no harm in that - but presenting it as a scientific argument is inappropriate.

[kezzafish]

so are you saying that you agree with the idea that mid corner (with CF) you are not in the same homognous gravitational field as you were on the straight part of the road? i don't quite understand your answer. Except for the "go ride" bit which i agree with however i find physics and discussions on it very interesting and love to shoot the shit with anyone who feels the same

[kezzafish]

maybe we should call COG the point of balance for this discussions sake?

[swbarnett]

From the "Feynman Lectures on Physics" by Richard Feynman*:

"The center of mass is sometimes called the center of gravity, for the reason that, in many cases, gravity may be considered uniform. ...In case the object is so large that the nonparallelism of the gravitational forces is significant, then the center where one must apply the balancing force is not simple to describe, and it departs slightly from the center of mass. That is why one must distinguish between the center of mass and the center of gravity."

I suspect that the CoG will move when a rider is in a corner for two reasons:

1. The Centripetal force acts like artificial gravity and alters the overall gravitational field (Earth's gravity + CF is equivalent to gravity that's not quite vertical)

2. The act of leaning into a corner will lower the overall mass and therefore the CoM.



*A very famous physicist of the last century.

[Mikkel]

From the "Feynman Lectures on Physics" by Richard Feynman*:

"The center of mass is sometimes called the center of gravity, for the reason that, in many cases, gravity may be considered uniform. ...In case the object is so large that the nonparallelism of the gravitational forces is significant, then the center where one must apply the balancing force is not simple to describe, and it departs slightly from the center of mass. That is why one must distinguish between the center of mass and the center of gravity."

I suspect that the CoG will move when a rider is in a corner for two reasons:

1. The Centripetal force acts like artificial gravity and alters the overall gravitational field (Earth's gravity + CF is equivalent to gravity that's not quite vertical)

2. The act of leaning into a corner will lower the overall mass and therefore the CoM.



*A very famous physicist of the last century.

Seriously, how the fuck can you quote Feynman and then dare to start your next sentence with "I suspect..."? ...and then continue to add such nonsense to the mix?

It is needless to say that the quote by Feynman is correct, he does however not mention the other extreme situation where the gravitational field has a large gradient - i.e. close to a black hole.

The difference in the spatial position of the COM compared to the COG is irrelevant in practicaly all and any classical mechanical systems - including riding a motorcycle. The origin of the whole COG versus COM debate is simply that the COM is what is of interest when we are discussing a mechanical system, not the COG (unless we are considering one of the situations mentioned above and consequently their locations differ).

[jrandom]

[YellowDog]

So the Propellar Heads win it then.

I did a very good slow speed motorcycle skills course a few months ago. It taught me how to use my body weight to counteract physics. At the end of the 3 days I was doing left and right low speed 360 turns on a steep incline. I now find that I crawl all over the bike to keep good balance, which is something I had previously struggled with.

[kezzafish]

Seriously, how the fuck can you quote Feynman and then dare to start your next sentence with "I suspect..."? ...and then continue to add such nonsense to the mix?

It is needless to say that the quote by Feynman is correct, he does however not mention the other extreme situation where the gravitational field has a large gradient - i.e. close to a black hole.

The difference in the spatial position of the COM compared to the COG is irrelevant in practicaly all and any classical mechanical systems - including riding a motorcycle. The origin of the whole COG versus COM debate is simply that the COM is what is of interest when we are discussing a mechanical system, not the COG (unless we are considering one of the situations mentioned above and consequently their locations differ).

if i said that the point of balance changes would you agree? i'd also like to point out that earlier on in the thread you were saying that the COM and COG are the same thing. And to your point about science defying common sense, i would say science defines common sense as common sense is just widely known science? i think your point is correct in a way because as you say, at no point in time does gravity change during cornering but it's effect is magnified. Agreed? i'm not saying i'm right and your wrong because i'm not certain in my belief and am interested in hearing anyone's take. do you agree that weighting the pegs is beneficial? i don't mean to wind you up and know how frustrating it is when you KNOW something as FACT and people don't believe you, but relax man

deleted

aye?

So the Propellar Heads win it then.

I did a very good slow speed motorcycle skills course a few months ago. It taught me how to use my body weight to counteract physics. At the end of the 3 days I was doing left and right low speed 360 turns on a steep incline. I now find that I crawl all over the bike to keep good balance, which is something I had previously struggled with.

does that mean me? (pp heads)

[swbarnett]

Seriously, how the fuck can you quote Feynman and then dare to start your next sentence with "I suspect..."? ...and then continue to add such nonsense to the mix?

Settle down. The quote was to illustrate that CoM and Cog are NOT the same thing but, as you say, for all practical purposes they can be considered as such.

It is needless to say that the quote by Feynman is correct, he does however not mention the other extreme situation where the gravitational field has a large gradient - i.e. close to a black hole.

I think this would be the same as a large object in a weaker field.

[swbarnett]

...use my body weight to counteract physics.

Sorry to be pedantic but you can't counteract physics. All you're doing is learning to work with it.

[swbarnett]

I was talking with my wife last night (Masters degree in Physics and a University lecturer). Her take is that Centripetal Force is not a force at all but an artefact of the cornering body's inertia (when a body is travelling at a finite velocity it will resist any attempt to change it's direction of travel). As such CF cannot alter the CoG.

[YellowDog]

Sorry to be pedantic but you can't counteract physics. All you're doing is learning to work with it.

You're not being pedantic.

Whilst you are correct in stating that you cannot counteract physics; what I am saying is that you can counteract or influence the resulting effect of physics on the bike if there was no change and in so doing change the resulting physics. But yes, it is still physics!

[Mikkel]

Settle down. The quote was to illustrate that CoM and Cog are NOT the same thing but, as you say, for all practical purposes they can be considered as such.

I accept that. But bringing a quote like that to the table kinda evokes certain expectations. Nothing personal at all mate.

I think this would be the same as a large object in a weaker field.

It has nothing to do with neither the magnitude nor the orientation of the gravitational field. Only the gradient in the gravitational field matters.
Anyway, both of the cases are extremities that we need not concern ourselves with. I am merely arguing semantics.

I was talking with my wife last night (Masters degree in Physics and a University lecturer). Her take is that Centripetal Force is not a force at all but an artefact of the cornering body's inertia (when a body is travelling at a finite velocity it will resist any attempt to change it's direction of travel). As such CF cannot alter the CoG.

Your wife is right on the money. Observed from outside an object needs to be affected by a centripetal force (a force that is orthogonal to the direction of travel) in order to perform a circular movement. The radius of the movement is proportional to the square of the velocity, proportional to the mass of the obejct in question and inversely proportional to the centripetal force.

If you perform a coordinate transformation and consider the object that performs the circular motion the origin of your coordinate system you will experience an artificial force equal, but anti-parallel, to the centripetal force - the centrifugal force.

And that is what I mean when I say science defies common sense. If it didn't we wouldn't have had to wait for close to 2000 years before a genius arrived that could do away with the common sense approach of aristolethean mechanics. Luckily Newton managed to achieve that, and even developed a language that could express these observations with the outmost of clarity (differential calculus - and yes, it is arguable whether Newton or Leibniz was the first).

[swbarnett]

I accept that. But bringing a quote like that to the table kinda evokes certain expectations. Nothing personal at all mate.

The quote was actually from a Wikipedia article. I thought I had a reasonable handle on the whole thing but it seems my understanding is a little rusty. My wife actually has a hard-bound copy of said lectures in her study.

As to the rest of what you've said, I know enough to see from this that you know what you're talking about (and certainly know more about this than I do). It makes for interesting reading.