Inertia and Friction Question

Yes, but what do you mean by rigid body simulator?

a simulator controlling the motion of rigid bodies like boxes under influences of forces.

For 2d it is easy to implement:
Each body you simulate(eg a ship) is assigned a position p (vector), orientation r (float), linear velocity v (vector) and angular velocity w(float).

Assuming a timestep dt you have the following update rules

p +=  v * dt , v = a * dt = f / m * dt where f is the force working on the ship

For the orientation the equations are a little bit more complex

here we have a force f working on a point r on the body. This means that we can compute a torque om the body as T = (r - r_cm)_x * f_y - f_x *  (r - r_cm)_y. Where r_cm is the position  of the center of mass of  our ship or rigid body

Then then orientation of the body can be found using the following update ruke

r = w * dt , w = T / I * dt

where i a number that tells how much the body resist orientation. A rotational version of mass and you can play with the values.

the orientation of the body can then be found by cos(r)

Um, that sounds about right but it looks confusing, could you possible simplify it to were a beginner in physics can understand, also make a legend of what the variables mean because halfway through I got lost.

Also I have it updating 180 frames/sec
and the movement update is done every 10 frames.

please tell exactly what you don't understand and i will try to clarify it

Both of the equations.

well all of equations

i'm gonna give a more indepth view of this.

Ship:
-Movement
newx = ship.position - Speed * -Cos((ship.degree + 90) / 360 * 2 * PI)
newy = ship.position - Speed * Sin((lastdegree + 90) / 360 * 2 * PI)
Uses x and y coordinates to determine next place it goes
This works even when throttle is off, has a slow down built in when throttle is off
When throttle is on current speed is increased to a maximum and when it isn't decreased to 0.

Except I would like it to have a sortof flight pattern as a game I played:
http://www.positech.co.uk/starminer/
Its 2d and has a nice engine for movement and I am trying to simulate that into my game.

-Turning
ship.degree = ship.degree (+/-) TurnRate
(+ for left, - for right)

Hope this helps more than my first example.

If the ship has its throttle off, e.g. "coasting", should turning it have an effect on the direction that it goes?

no

Okay, then the best way to maintain the ship's position and velocity is with a vector and a point.  The point represents the position, and the vector represents the speed and direction of the ship's movement.  This way, whenever your ship has the throttle on, for each time unit that the throttle is on, given the current direction, you add the vectors and have your new velocity vector.  Calculating the new position based on the vector should be trivial.

um, what do you mean by trivial?
and is there any equations I could use?

guitaristx > If the ship has its throttle off, e.g. "coasting", should turning it have an effect on the direction that it goes?
CoolAzura > no
Why not?  Is the ship completely symmetrical in its response to friction?

it slows down on the way that it is going when coasting.

but can turn with no effect on which way it goes until throttle is applied.

So what happens if the throttle is turned off while the rudder is applied?
How does turning affect the way it goes when the throttle is applied?

when rudder or turn is applied the ship rotates counterclockwise or clockwise, while drifting towards the position it was going before the thrust was turned off.


when throttle is applied it moves towards the current angle of the ship.
using these 2 formulas:
newx = ship.x - Speed * -Cos((ship.degree + 90) / 360 * 2 * PI)
newy = ship.y - Speed * Sin((lastdegree + 90) / 360 * 2 * PI)


these use a circle of where it is susposed to move, using the current degree or angle of the ship and where the ending point of a line will be on any part of the circle.

Most ships which sail in water are longer than they are wide, so that when they drift diagonally, friction exerts more force on its sides than on its ends, which causes the direction it is going to change.
Is your ship somehow different?
Is your ship a space ship?
If so, there should be no friction to slow it down.

Well it is a space ship, and I know that there should be no friction, but I am just making it slow down because I think it would not be a great aspect for it to keep moving.

In that case, it is probably easiest to use equations like
newx = ship.x + velocity.x
newy = ship.y + velocity.y

velocity.x = velocity.x*(1-1/friction) + throttle* Sin((ship.degree) / 180 * PI)
velocity.y = velocity.y*(1-1/friction) + throttle* Cos((ship.degree) / 180 * PI)

Unless you also want to account for the deceasing mass of the ship as fuel is expended.

well, I see how you get velocity, but what does velocity mean?

and does that account for inertia as well?

velocity is a vector describing rate of change in position,  I assume (ship.x,ship.y) and (newx,newy) are successive position of the ship

Inertia wants to keep the velocity constant.  (so without throttle or friction, it won't change)

well for some reason it makes my ship fly very fast and in the wrong direction

I think 1-1/friction was not the best way to express that, 1-friction would have made more sense.

Where is your ship pointing when ship.degree=0?

North

one sec i am trying to work with this.

Is north the same as +y ?

What direction is ship.degree=90?

no -y


North: -y
South: +y
East: -x
West: +x

West

Well I fixed the formula to work, but it still makes the ship move in the direction that it is turning even when coasting.

That's an unusual standard, but in that case, try
velocity.x = velocity.x*(1-friction) + throttle* Sin((ship.degree) / 180 * PI)
velocity.y = velocity.y*(1-friction) + throttle* -Cos((ship.degree) / 180 * PI)

By coasting, do you mean with throttle=0?
In that case velocity should remain in the same direction, and only slow by friction (which should be between 0 and 1)

hmm, let me think

when throttle is applied I have the current speed go up to a maximum speed.

then by it slowly goes down depending on friction.. what should I use as throttle?

Current Speed
or
Max Speed

ok I got friction to work, but now I need to apply Inertia

so when it turns it doesn't just turn right away but has a wide turn

for example:
When driving a car and you turn you have inertia and the car is still trying to move in the direction that it was.

It will have a wide turn, unless you have too much throttle.
Throttle should generally be small compared to speed.

Wait, what do you mean by current speed?
I don't represent speed explicitly in the equations, but you can calculate it as
sqrt(velocity.x*velocity.x+velocity.y*velocity.y)

oops sorry I said it wrong.

Ok I will explain:

Ship is pointing at 0 degrees when it turns to (lets say 60 degrees)
And we are using a reference point of a circle that is around the ship with the length of how fast the ship is going.

Now when the turn is applied it will go to some other points before it actually starts going at the 60 degree mark.

Tell me if you understand, if not I will try to simplify and make it clearer

well I already have the movement working right so no need to change that exept for inertia.

Starting at velocity.x=velocity.y=0 the ship will not move, no matter which way it turns, untill you apply throttle.
While throttle is applied, your speed will gradually increase.
When throttle goes back to 0, speed will gradually decrease under friction.

I see, but do you know how I could add the inertia?

Velocity does obey inertia.  It remains constant unless acted upon by a force (friction or throttle), and slowly changes as long as that force is applied.
To increase inertia, add more mass, and use (friction/mass) and (throttle/mass)

so in the equations it would be:

velocity.x = velocity.x*(1-(friction/mass)) + (throttle/mass)* Sin((ship.degree) / 180 * PI)
velocity.y = velocity.y*(1-(friction/mass)) ) + (throttle/mass)* -Cos((ship.degree) / 180 * PI)

?

I see, let me try these new things out.

It may be more clear to write

ship.x = ship.x + velocity.x
ship.y = ship.y + velocity.y

could you possible download a game where i got this idea from, http://www.positech.co.uk/starminer/
This is how i would like the ship to fly.
but for some reason the formula you gave me doesn't quite work out the way i would like it to.

Please try take a look at the way the ship flies in that game.