Almost Physics III – Equilibrium Forces (interesting part)

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momo
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My posts are covering basic physics laws, I hope readers are aware that I am still newbie and that what you read in my post is far away to replace the Marshals, Explorers and Rescue team teaching advises through the organized trips. I discourage readers to keep the content of this posts as a standard when driving as the desert may be an hazardous terrain, the beautiful straight dune that you might see on a drawing maybe completely different in real. So please, what you read is just there for general culture.


We have seen previously gravitational force and normal force, now we are ready to trace all the forces which are dependent of these two essential forces.

[ATTACH=CONFIG]1893[/ATTACH]

All the arrows mentioned on the schema are forces vectors, meaning that they have directions and values.

We start with Force 1


F1
This force as mentioned in previous thread, is the result of the difference between gravitational force Fg and Normal force FN
This force brings our vehicle downhill. (on first schema)

F2
Force 2, at stop, is equal to F1 but opposite in direction. Actually, this is the resistance of the tires to avoid our vehicle going backward (first image). We call it Friction force.
The only problem with friction force, is that it has some limits. Friction depend of the combination of the type of tires hardness and shape and the surface on which it sit. In our case the surface is sand, so the friction is decreased (slipping effect).
[ATTACH=CONFIG]1894[/ATTACH]
In result F2 is reduced, and this effect is more accentuated under motion. So imagine we lose part of F2, F1 will be higher and win. This will drag the vehicle down.
On a dune side stop, if we lose F2, the vehicle tires will start slipping downward creating a resistance = rotation point (red point)
This resistance point is there simply because your tires are not parallel to the movement direction and F1 will benefit of that resistance point to make the car roll-over.
[ATTACH=CONFIG]1895[/ATTACH]


F3
We can understand this force by logic, if Fn tries to make you fly, F3 will keeps you on the ground. So these forces are equal in value but opposite direction.
As we know, if we increase the dune approach angle θ, Fn is reduced wich will make F1 higher. At this moment the car is in motion and will start slipping downward so F2 decreases because of the tire friction.
F3 at this point take different value and direction. And this situation will eases the roll-over because F3 will be pointed to the rotation red point and so it lose its duty as a force, we will remain with F1 which is much higher than F2, and so F1 wins. Roll-over occurs.
[ATTACH=CONFIG]1895[/ATTACH]

This video below (50% slow playback) includes an example of F1, the red points on which the car rolls over and the track chosen by mistake:


[video=youtube;zn5QIQ25nhA][/video]


To conclude:

- For non experienced drivers don't try to climb an extreme approach angle dune. Because you will put heavy load on the engine as it should do the hard work to provide more forces than F1 which is huge at high angle. Remembering that your tires have limited friction so if your engine has 300 horse power don't be attempted because it will lose part of it in the tire friction.

- If you find your self side slipping on a dune, keep your tires rotating it will decrease the friction of your tires on the sand and increase the slipping effect and that avoid you temporary the resistant rotation point (red point) which is a key point for roll-overs. Meanwhile you should turn your steering to the F1 force direction. And try to be parallel to F1 as much as you can to be in the safe side. If you bring your car almost parallel to F1 at least (50%) and you stack there because of very soft sand, stop at this point and ask for assistance.

- Whatever you do on a sand dune, keep in mind that there are few forces which act opposite your direction, but in the same time they can recover you. As example, if you go upward a dune, and your vehicle cannot do it until the crest and start digging, don't insist. Most probably F1 will help you to go backward and try a different way.

Note: Until this point, with the respect of club Founders, Marshals and Leaders, given advises are from a physics point of view, they can only add a theoretical base to what you will learn with Marshals and Leader from years of experience on the terrain.


Regards,
Mohanned

Any correction or clarification are most welcome.
Mark B
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Mono.

I know that centrifugal forces play a role in keeping a motor cycle upright while in motion. I see two forms of centrifugal forces at play when drifting. The curvature of the dune and the rotation of the tires. What is the overall contribution of each to stability. For example, will spinning your tires in a drift give you measurable stability.
momo
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Mark B;35425 wrote:Mono.

I know that centrifugal forces play a role in keeping a motor cycle upright while in motion. I see two forms of centrifugal forces at play when drifting. The curvature of the dune and the rotation of the tires. What is the overall contribution of each to stability. For example, will spinning your tires in a drift give you measurable stability.
Thanks Mark B for the additional informations, I am just trying to keep the threads organised and not mix everything in one thread. I will write a post about the topic you mentioned. Regards,
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Wow i hated physics in high school but this stuff is golden :D understood each and every word of it :D
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caprihorse
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In many points you are wrong and you are forgetting about other physical elements which exists in our world. You didn't mention Center of Gravity [The center of gravity (CG) of an object is the balance point around which there are equal moment arms of length times weight. The object can act as if all its weight was concentrated at the CG.]

Free rotation of an object is always around its center of gravity and that an object will tip over when the center of gravity lies outside the supporting base of the object. Any object can tip over, when CG lies outside the supporting base of the object. See it here in this picture and look at force F8 line.

[ATTACH=CONFIG]1921[/ATTACH]

In this picture you can see that FJ's supporting base is outside F8, where Patrol's supporting base is not influenced heavily by F8.

More dangerous is, if somebody does increase CG by adding additional weight on the top and therefore increasing CG distance from supporting base, again influenced by F8.

[ATTACH=CONFIG]1922[/ATTACH]

In this case the potential of roll over is higher, mainly when the car is standing, if somebody hits the brakes at wrong place and time. The car's moving force to the front direction will reduce influence of F8.

So once you'll come into such delicate situation, keep moving down, reducing the distance between CG and supporting base, to be on the safe side.
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caprihorse
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In many points you are wrong and you are forgetting about other physical elements which exists in our world. You didn't mention Center of Gravity [The center of gravity (CG) of an object is the balance point around which there are equal moment arms of length times weight. The object can act as if all its weight was concentrated at the CG.]

Free rotation of an object is always around its center of gravity and that an object will tip over when the center of gravity lies outside the supporting base of the object. Any object can tip over, when CG lies outside the supporting base of the object. See it here in this picture and look at force F8 line.

[ATTACH=CONFIG]1923[/ATTACH]

In this picture you can see that FJ's supporting base is outside F8, where Patrol's supporting base is not influenced heavily by F8.

More dangerous is, if somebody does increase CG by adding additional weight on the top and therefore increasing CG distance from supporting base, again influenced by F8.

[ATTACH=CONFIG]1924[/ATTACH]

In this case the potential of roll over is higher, mainly when the car is standing, if somebody hits the brakes at wrong place and time. The car's moving force to the front direction will reduce influence of F8.

So once you'll come into such delicate situation, keep moving down, reducing the distance between CG and supporting base, to be on the safe side.
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caprihorse
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You are forgetting about other physical elements which exists in our world. You didn't mention Center of Gravity [The center of gravity (CG) of an object is the balance point around which there are equal moment arms of length times weight. The object can act as if all its weight was concentrated at the CG.]

Free rotation of an object is always around its center of gravity and that an object will tip over when the center of gravity lies outside the supporting base of the object. Any object can tip over, when CG lies outside the supporting base of the object. See it here in this picture and look at force Fg line.

Image

In this picture you can see that FJ's supporting base is outside Fg, where Patrol's supporting base is not influenced heavily by Fg.

More dangerous is, if somebody does increase CG by adding additional weight on the top and therefore increasing CG distance from supporting base, again influenced by Fg.

Image

In this case the potential of roll over is higher, mainly when the car is standing, if somebody hits the brakes at wrong place and time. The car's moving force to the front direction will reduce influence of Fg.

So once you'll come into such delicate situation, keep moving down, reducing the distance between CG and supporting base, to be on the safe side.

In your video (good job), it can be seen that the driver at 0:21 was trying to fight Fg and was steering up. This caused him a certain important time loss, where Fg was increasing. Another fatal thing was, that sopped the car for few milliseconds and in that moment CG was outside supporting base and Fg acted with the highest possible force. Instead of driving up, the driver should steer down without stopping and therefore decrease the distance between CG and supporting base and not waking up MDG.
momo
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Thank you very much caprihorse for correcting and for the additional informations. In french we say "cerise sur le gâteau" meaning cherry on the cake. In other words for equilibrium forces you just mentioned the essential point where all these forces are acting on. I just would like to bring your attention to F8, actually on the image due to pixel problem it shows F8 but actually it is Fg gravitational force.

Regards,

Momo
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caprihorse
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MOMO;35443 wrote:Thank you very much caprihorse for correcting and for the additional informations. In french we say "cerise sur le gâteau" meaning cherry on the cake. In other words for equilibrium forces you just mentioned the essential point where all these forces are acting on. I just would like to bring your attention to F8, actually on the image due to pixel problem it shows F8 but actually it is Fg gravitational force.

Regards,

Momo
Thanks, re-edited.
Unleashed_lee
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Thanks for the thread, this to me is the most scariest part of off roading, at the beginning i always had this fear in me on how to approach a dune and I won't say it has gone away now, but I certainly got more experience. Still i have lots to learn.
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