math help

I cant figure out how do i find the friction vector R (not just its length) in 3D when i know normal vector N, force vector F and friction constant u. And ofc i know object mass too

To make it simple:
N = object weight = mg
f= Friction force = nN where n is a constant value (0.25 - 0.45 could be used) and its direction is the opposite to the force movement. That's all :)

ye moyack i know that. But how do i get the friction vector using vector math... that is wat i wanna know. I dont wanna use any Cos,Sin,SquareRoot or watever ^^

and vex: posting tat think doesnt help much, so no thnx. Cus i think i can use google too, but mostly what i find is alot of explaination... i just want summary like, not the whole thing.
And besides, in that page it says nothing about vector... finsing friction is easy, but i just dont know how to Inverse the force vector and make it same length as normal vector N

...

Just in the first line of the wikipedia article "Opposing to the force..."

then moyack explained already.

Direction of friction = opposite (Aka (-Fx,-Fy ,-Fz) ?)
Length of friction = you already got it.

If you have a vector and wish to change its length, what do you do?

A vector is defined by its magnitude and direction, and I gave you both... you don't need anything else.

1. A vector is not a magnitude and a direction until you define a dot product
2. Of course we do have an obvious choice of dot product here
3. The direction of kinetic friction will be opposite to the direction of motion
4. The length will be the length of the normal force

The relevant equations for you to solve are:
f = N + Fe + Fr= ma
where N is the vector normal force, Fe is the external force, Fr is friction, and a is the acceleration
Fr = v_hat |N| mu
N . v_hat = 0
n_hat is the direction of the normal vector, v_hat is the velocity and mu is the
That's a coupled set of 7 equations which you want to solve for a, |N|and Fr. Try taking dot/cross products of them with N.

OFC i know that the friction vector is opose to force vector, and the length is length of normal vector. But this deosnt solve anything.
I want to get the friction vector using vector math without using Sin/Cos/root.
Which means i need a formula.
If i know magnitude and direction, a vector doesnt appear automatically -.-
I only know how to reverse the force vector, but then the length isnt same as normal vector, so how do i change its length to that of normal vector using vector math.

I know of a way to solve it already, just that method require squareroot... which is slow for wc3, so i dont want

You want to do vector maths...without trig...? Sin/Cos are incredibly fast (faster than a blank function call), so just use them.

i said no Sin/cis/QuareRoot
SIn/Cos are damn slow

Ok MAd.... I suppose your vector system can scale vector.... so let's scale vectors.

1. Take the movement vector F and get a unitary vector with the same direction
2. Then scale it by -1
3. Then scale it by nmg where n is friction coefficient, m is the object mass and g is gravity
4. and we will have the friction vector. So the final scale will be: -nmg

what is unitary vector

edit: a nvm got it, brilliant :D damn why didnt i think of using unitary vectors :P

problem again...

How do i get unitary vector. I can only think of one way, but that way i need squareroot

Another thing to consider is that the friction must only be applied to the component of the velocity that is parallel to the ground...

i know that too grim

but as i updated the post above, how to get unitary vector...

My version of friction requires squareroot as well, there's no way around it.
Just make sure that if an object has 0 friction it skips that part of the code. Then the user can decide if they want to use it or not.