# Physics: Newtonian

These are the “normal” particle physics. Particles start their life with the specified initial velocities and angular velocities, and move according to forces. The response to environment and to forces is computed differently, according to any given integrator chosen by the animator.

## Integrators

Integrators are a set of mathematical methods available to calculate the movement of particles. The following guidelines will help to choose a proper integrator, according to the behavior aimed at by the animator.

• Euler: Also known as “Forward Euler”. Simplest integrator. Very fast but also with less exact results. If no dampening is used, particles get more and more energy over time. For example, bouncing particles will bounce higher and higher each time. Should not be confused with “Backward Euler” (not implemented) which has the opposite feature, energies decrease over time, even with no dampening. Use this integrator for short simulations or simulations with a lot of dampening where speedy calculations is more important than accuracy.
• Midpoint: Also known as “2nd order Runge-Kutta”. Slower than Euler but much more stable. If the acceleration is constant (no drag for example), it is energy conservative. It should be noted that in example of the bouncing particles, the particles might bounce higher than they started once in a while, but this is not a trend. This integrator is a generally good integrator for use in most cases.
• RK4: Short for “4th order Runge-Kutta”. Similar to Midpoint but slower and in most cases more accurate. It is energy conservative even if the acceleration is not constant. Only needed in complex simulations where Midpoint is found not to be accurate enough.

## Initial velocity

The initial velocity of particles can be set through different parameters, based on the type of the particle system (see Particle System tab). If the particle system type is Emitter or Hair, then the following parameters give the particle an initial velocity in the direction of…

• Object: …the emitter objects movement (i.e. let the object give the particle a starting speed).
• Normal: …the emitter’s surface normals (i.e. let the surface normal give the particle a starting speed).
• Random: …a random vector (i.e. give the starting speed a random variation in direction and in value. You can use a texture to only change the value, see Controlling Emission, Interaction and Time).
• Tan & Rot: …a tangential vector along the surface, rotated by Rot.
• Tan: Let the tangent speed give the particle a starting speed.
• Rot: Rotates the surface tangent.

If the particle system type is Reactor, then the following parameters give the particle an initial velocity in the direction of…

• Particle: …the target particles velocity (i.e. let the target particle give the particle a starting speed).
• Reactor: …a vector away from the target particles location at the time of the reaction (i.e. let the vector away from the target particles location give the particle a starting speed).

## Rotation

These parameters specify how the individual particles are rotated during their travel. To visualize the rotation of a particle you should choose visualization type Axis in the Visualization panel and increase the Draw Size.

• Dynamic: If enabled, only initializes particles to the wanted rotation and angular velocity and let’s physics handle the rest. Particles then change their angular velocity if they collide with other objects (like in the real world due to friction between the colliding surfaces). Otherwise the angular velocity is predetermined at all times (i.e. set rotation to dynamic/constant).
• Rotation: Sets the initial rotation of the particle by aligning the x-axis in the direction of…
• None: …the global x-axis.
• Normal: …the emitter’s surface normal.
• Velocity: …the particle’s initial velocity.
• Global X/Global Y/Global Z: …one of the global axes.
• Object X/Object Y/Object Z: …one of the emitter object axes.
• Random: Randomizes rotation.
• Phase/Rand: Initial rotation phase, Rand allows a random variation of the Phase.
• Angular v: The magnitude of angular velocity, the dropdown specifies the axis of angular velocity to be…
• None: …a zero vector (no rotation).
• Spin: …the particles velocity vector.
• Random: …a random vector.

If you use a Curve Guide and wan’t the particles to follow the curve, you have to set Angular Velocity to Spin and leave the rotation on Constant (i.e. don’t turn on Dynamic). Curve Follow does not work for particles.

## Global effects

These parameters specify global physical factors that accelerate or decelerate particles velocity. Useful when simulating various phenomenon like gravity, air-drag, friction and such. Other, more complicated or localized forces can be created with Force Fields.

• AccX, AccY and AccZ: An acceleration in the direction of the global axes. Use this to implement gravity by setting AccZ to a negative value, for example.
• Drag: A force that reduces particle velocity in relation to it’s speed and size (useful in order to simulate Air-Drag or Water-Drag).
• Brown: A random force that changes from frame to frame. Simulates Brownian movement which is an effect seen on (very) small particles where forces from individual molecules are unbalanced over time. This is nice to simulate small, random wind forces.
• Damp: Reduces particle velocity (deceleration, friction, dampening).