Single pendulum done with reaction forces instead of the analytic equations of motion as in PendulumSim. This is similar to how the rigid body physics engine in ContactSim calculates forces, but this is specific to only this particular single pendulum scenario.

The pendulum is regarded as a rigid body consisting of a uniform disk at end of a massless rigid arm. We find the reaction forces by solving a matrix equation, following the steps shown at


The pivot is fixed at the origin.

  • (x,y) = center of disk
  • w = angle of pendulum

Note that w is the angle of the pendulum in relation to the pivot point, which happens to also correspond to the angle of the disk rigid body (perhaps adding a constant).

TO DO make dragable for setting start angle?

Hierarchy (view full)




initialState_: null | number[] = null

Initial values.

radius_: number

radius of rigid body pendulum disk


  • Adds the given Observer to this Subject's list of Observers, so that the Observer will be notified of changes in this Subject. An Observer may call Subject.addObserver during its observe method.


    Returns void

  • Defines the differential equations of this ODESim; for an input set of variables, returns the current rate of change for each variable (the first derivative of each variable with respect to time).

    The timeStep is the time since the state variables were last fully calculated, which can be and often is zero. The current time can be regarded as getTime() + timeStep. The input variables correspond to the Simulation state at that time. Note that timeStep is different from the time step used to advance the Simulation (as in AdvanceStrategy.advance). The timeStep is typically used when finding collisions in CollisionSim.findCollisions.


    • vars: number[]

      the current array of state variables (input), corresponding to the state at getTime() + timeStep

    • change: number[]

      array of change rates for each variable (output), all values are zero on entry.

    • _timeStep: number

      the current time step (might be zero)

    Returns null | object

    null if the evaluation succeeds, otherwise an object relating to the error that occurred. The change array contains the output results.

  • Sets whether this Subject will broadcast events, typically used to temporarily disable broadcasting. Intended to be used in situations where a subclass overrides a method that broadcasts an event. This allows the subclass to prevent the superclass broadcasting that event, so that the subclass can broadcast the event when the method is completed.


    • value: boolean

      whether this Subject should broadcast events

    Returns boolean

    the previous value

  • Returns a minimal string representation of this object, usually giving just identity information like the class name and name of the object.

    For an object whose main purpose is to represent another Printable object, it is recommended to include the result of calling toStringShort on that other object. For example, calling toStringShort() on a DisplayShape might return something like this:


    Returns string

    a minimal string representation of this object.

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