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simotix

Bouncing a ball off a plane

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simotix    142
I am creating a simulation of a sphere (ball) bouncing on a plane, then rolling until it stops. However, I am having dificulty having the sphere actually stop due to friction. I believe it is because of my unstable friction state, however, I will post all of my details case I am wrong.
1) Check sphere and plane collision

     If there was no collision
          Compute total force -> ball.TotalForces = ball.force + ball.weight + ball.dragForce
          Compute acceleration -> ball.linearAcceleration = ball.TotalForces / ball.mass
          Compute the velocity -> ball.linearVelocity = ball.linearVelocity + ball.linearAcceleration * dt

     If there was collision
          Compute linear velocity after contact -> ball.linearVelocity = -ball.coefficientOfRestitution * ball.normalVelocity + ball.tangentialVelocity
Up to this point it has been rather generic, this is basically just general physics. It is when I have resting contact that I am not slowing down the force. The force that I need to be slowing down is ball.force, which is initially set to (100, 400, 0)
     If there was resting contact
         Check to see if it is in stable static state -> If ball.linearVelocity.Norm == 0
              Calculate static friction -> staticFriction = ball.StaticFrictionCoefficient * ball.normalForce.Norm * plane.normal.normalize
              Set the friction force to static friction -> ball.frictionForce = staticFriction
              Set the linear velocity to zero -> ball.linearVelocity = (0, 0, 0)
              Set the linear acceleration to zero -> ball.linearAcceleration = (0, 0, 0)
              Compute the total force applied to the ball -> ball.TotalForces = ball.weight + ball.normalForce

         Check to see if it is in dynamic state -> if ball.linearVelocity.Norm > 0 && force.norm >= staticFriction.norm
              Compute Kinetic friction -> kineticFriction = -ball.DynamicFrictionCoefficient * ball.normalForce.norm * linearVelocity.normalize
              Set the friction force to kineticFriction -> ball.frictionForce = kineticFriction
              Compute the total force -> ball.totalForces = ball.force + ball.weight + ball.normalForce + ball.frictionForce + ball.dragForce
              Compute the linear acceleration of the ball -> ball.linearAcceleration = ball.Totalforces / ball.mass
              Compute the linear velocity of the ball -> ball.linearVelocity = ball.linearVelocity + ball.linearAcceleration * dt;

         Check to see if it is in unstable state -> if ball.linearVelocity.Norm == 0 && ball.force.norm > staticFriction.norm
              Compute static friction in the opposite direction of the contact normal(***) -> staticFriction = -ball.StaticFrictionCoefficient * ball.normalForce.Norm * plane.normal.normalize
              Compute the sliding factor to make the transition from static to dynamic friction state -> slidingFactor = ball.force.norm / ball.frictionForce.norm
              Compute a small velocity to break the transition from static friction to dynamic friction -> ball.linearVelocity = slidingFactor * ball.force.normalize
              Compute kinetic friction -> ball.frictionForce = -ball.DynamicFrictionCoefficient * ball.force.norm * ball.linearVelocity
              Compute the total forces on the ball -> ball.totalForces = ball.force + ball.weight + ball.normalForce + ball.frictionForce + ball.dragForce
              Compute the linear acceleration -> ball.linearAcceleration = ball.totalForces / ball.mass
              Compute the linear velocity = ball.linearVelocity = ball.linearVelocity + ball.linearAcceleration * dt
After all of the collision reponses are done, I will just pdate the ball
2) Update the ball
	ball.speed = ball.linearVelocity.norm
	ball.dragForce = -.5f * ball.rho * ball.speed * ball.dragCoefficient * ball.area * ball.linearVelocity
	ball.position = ball.position + ball.linearVelocity * dt

One thing I notice is that the first calculate in the unstable state is of static friction (marked with the ***). I was unable to find a calculate that actually applies that static friction, even though I know it is required to be calulcated in that state. The other problem I ran into is that just ball.force never gets changed, yet that is that applied force. How can I have the ball.force slow down, am I applying a calculation wrong? I have posted about this in the past, but my question and details were unclear so I figured I would make a new post.

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bzroom    647
I want to help, but i dont even know where to begin. Can you refactor your question to make it easier for me to answer?

One thing i kind of wanted to mention, but dont know if it's what you're asking. When something is slowing down, due to resistance, you need to check when the deceleration would exceed the required acceleration to stop the object.

const float dec = #;

float actualDec;

if (abs(velocity) < dec * dt) actualDec = -velocity / dt;
else actualDec = -sign(velocity) * dec;

velocity += actualDec * dt; //will come to zero.


For static friction, there is no force to compute. If the object is determined to be in static friction then velocity is zero. No forces will be integrated into that velocity.

But basically you'll compute the max static friction force limit. And as long as all external forces stay below that, they will be zeroed out when integrating the velocity, the velocity will not be changed away from zero.

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