I''ve been goofing around with some gravity and air resistence equations on a small test app. It''s actually pretty fun, changing the gravity and air density and all. However, I think I need something to caculate rolling resistence. Does anyone have an equations for that? Just something simple, it doesn''t need to be too complex. Also, are there any resources on the net that give real world examples of what to plug into these equations? For example, rolling resistence of different wheels, coefficients of drag for different objects, that sort of thing?

Methinks it''d be better using trial and error until you get something that just "looks right".

By "rolling resistance", I assume you mean friction, which is a force in the opposite direction to the velocity and centred on the point of contact between the two objects. The magnitude is relative to the velocity of the objects and the coefficient of friction of the two objects.

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two types of friction. static and kinetic. rolling friction tends to be equal to static friction.

kinetic friction is used for sliding and it usually is not perportional to velocity. its pretty constant as a function of velocity just less then static friction.

both are perportional to the downward force on the normal. that is cars that weigh more have more effect from static/kinetic friction. a heavier box has more effect from static/kinetic friction.

air resistance is different cause it involves compression. the faster you go the more compressed your front side air is. also the faster you go the more backside vacuum you get. there isnt a perfect equation for that. but it is a function of cross section and velocity.









[edited by - declspec on May 11, 2002 6:24:04 AM]

Get a physic book.

there are three types of friction:
static friction, dynamic friction and rolling friction

rolling friction isn''t the same like static, more like dynamic.

static friction it what you see when a corpse is lying on the ground and you want to push it. You have to bring a force before it moves at all.

dynamic: If the corpse finally moves, the force of friction is some lower.

rolling: just like dynamic, but with a rolling corpse.

Fiction is dependent from the two materials touching, and the force in the vertical direction to the touching plane (that will be something like: weight * gravitation * dotproduct(planenormal, ZAxis))
The size of the surface isn''t interesting, the bigger the surface, the less the force per cm², but then you have more cm², so finally force stays the same.

there are friction coefficients for each of the three friction types and two materials touching each other.

so finally your force of moving your corpse will be reduced by
FrictionCoefficient * (Force part directed vertical to "sliding plane")

Hope this helped.

@declspec:
air friction also is a simple formula.
a = -k v²
(where a is the breaking force)
where v is the actual speed of the object and k is the air friction coefficient (again depending on two factors, the mediun, like air or any gas mixture, and the shape of the corpse).
Looking on that formula you will notice each body has a maximum free falling speed, where:
-k v² = g
solving this to v you will get
v = sqrt (g / -k)
in vakuum that k is zero, so there is no air friction at all and you don't get a maximum speed, because teh corpse will speed up forever... (at least until it impacts to the ground)

edit: forgot to mention what a stands for

[edited by - OmniBrain on May 11, 2002 7:57:59 AM]

your right. im not an engineer. i took honors physics for scientist but we didnt deal with rolling friction. so i popped open my book. had a slight remark in the optional part bout microscopic basis for friction. said that rolling friction was less then static friction because *theory* contact welds are being pulled apart rather then sheered.

ok cartoon theories aside. key comfirmation: rolling friction is less then static friction; ergo they arnt the same.

as for the air resistance. ya thats good but it breaks down. some cross sections/velocity/densities v^3 becomes more appropriate.

both are deep into the "Aprroximation" side. neither is exact. in the way that frictional equations tend to approximate fairly exactly. atleast thats what i was thinking when i made the post. but hey im not an engineer