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jsn_segal

Friction Problem

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I've been trying to convert my game's physics system to be frame-independent, but ran into a problem with resistive forces (friction, drag, etc.). Acceleration due to resistive forces always ends up being several thousand times what it should be, overcoming both current velocity and acceleration (ex: if velocity = 10 and standard acceleration = 1, then acceleration due to friction might be in the vicinity of -7000). I've included a copy of my "vehicle" class (written in Java) below, simplified operate in only one dimension. I think I made some sort of mistake either in the way I calculate the vehicle's drag coefficient in the constructor, or the way I integrate resistive force in the "updatePhysics" method, but I'm having trouble working it out. I'd appreciate any suggestions anyone can offer.
 

class Vehicle 
{
	float  maxSpeed = 0;
	double engineAcceleration = 0;
	float  xDragCoefficient = 0;
	
	private float xPosition = 0;
	private float xSpeed = 0;
	private float xAcceleration = 0;
	private float xResistance = 0;
	
	public Vehicle(int newMaxSpeed, int newEngineAcceleration)
	{
		//This is the maximum speed this vehicle should attain under its own power (per millisecond)
		maxSpeed = new Double(newMaxSpeed*0.001).floatValue();
		
		//This is the acceleration caused by the force of the vehicle's engine (per millisecond)  
 		engineAcceleration = new Double(newEngineAcceleration*0.001).floatValue();

		//This represents the drag coefficient of the vehicle's "front" cross-section.
		//It should keep the vehicle's velocity (xSpeed) from exceeding maxSpeed under 
		//engine power alone.
		xDragCoefficient = new Double(2*engineAcceleration/(2*maxSpeed + engineAcceleration)).floatValue();
	}
	
	public void updateVehicle()
	{
		//The engine is accelerating the vehicle forward, possibly in addition to external forces
		xAcceleration += engineAcceleration;
		
		//The motion of the vehicle is resisted by its cross-sectional drag, possibly in addition to other resistive forces
		xResistance += xDragCoefficient;	
		
		updatePhysics();			
	}


	private void updatePhysics()
	{
		//Get the number of milliseconds that have passed since the last update cycle
		int time = Test.getElapsedTime();
		
		//Temporarily store the initial velocity
		float xSpeedInitial = xSpeed;
		
		//Integrate the acceleration due to standard forces (acceleration is constant over a single cycle)
		xSpeed += xAcceleration*time;

		//Determine the average velocity over this cycle in absence of resistive forces
		float averageXSpeed = (xSpeedInitial + xSpeed)/2;
		
		//Determine the average resistive acceleration over this cycle
		float resistanceXAcceleration = (-xResistance*(averageXSpeed)*time);
			
		//Integrate the acceleration due to resistive forces
		xSpeed += resistanceXAcceleration*time;
		
		//Update the possition by integrating the average speed, now that resistive forces are accounted for
		xPosition += ((xSpeedInitial + xSpeed)/2)*time;
		
		//Set Acceleration and Resistance to zero in preparation for the next cycle
		xAcceleration = 0;
		xResistance = 0;
	}
}



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Why not do your updatePhysics function something like this?


private void updatePhysics()
{
//Get the number of milliseconds that have passed since the last update cycle
int time = Test.getElapsedTime();

xAcceleration = engineAcceleration - (xDragCoefficient * xSpeed) / mass

xSpeed += xAcceleration*time;

xPosition += (xSpeed)*time;
}


That's just using simple euler integration (this may not suffice, it depends upon what you're actually doing, if it's a simple physics simulation you may get away with it) and models the resistive force experienced by the car as F = kv where k is a constant (your xDragCoefficient) and v is the current speed).

You'll also need to calculate your drag coefficient differently and if I've got my maths right (it's been a while since I've done anything like that so I may have got it wrong) you can do it like this:


xDragCoefficient = new Double(mass * engineAcceleration / maxSpeed)


Oh and to convert your acceleration to milliseconds you need to multiply by 0.0012

Also why are xAcceleration and xResistance class variables? Why aren't they just local variables in updatePhysics?

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