Almost Physics III – Equilibrium Forces (interesting part)
Posted: Tue Apr 29, 2014 5:28 pm
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.
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.