p2s/src/objects/RotationalSpring.js

A JavaScript 2D physics engine.

var Spring = require('./Spring');

module.exports = RotationalSpring;

/**
 * A rotational spring, connecting two bodies rotation. This spring explicitly adds angularForce (torque) to the bodies.
 *
 * The spring can be combined with a {{#crossLink "RevoluteConstraint"}}{{/crossLink}} to make, for example, a mouse trap.
 *
 * @class RotationalSpring
 * @extends Spring
 * @constructor
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @param {Object} [options]
 * @param {number} [options.restAngle] The relative angle of bodies at which the spring is at rest. If not given, it's set to the current relative angle between the bodies.
 * @param {number} [options.stiffness=100] Spring constant (see Hookes Law). A number >= 0.
 * @param {number} [options.damping=1] A number >= 0.
 *
 * @example
 *     var spring = new RotationalSpring(bodyA, bodyB, {
 *         stiffness: 100,
 *         damping: 1
 *     });
 *     world.addSpring(spring);
 */
function RotationalSpring(bodyA, bodyB, options){
    options = options || {};

    Spring.call(this, bodyA, bodyB, options);

    /**
     * Rest angle of the spring.
     * @property restAngle
     * @type {number}
     */
    this.restAngle = options.restAngle !== undefined ? options.restAngle : bodyB.angle - bodyA.angle;
}
RotationalSpring.prototype = new Spring();
RotationalSpring.prototype.constructor = RotationalSpring;

/**
 * Apply the spring force to the connected bodies.
 * @method applyForce
 */
RotationalSpring.prototype.applyForce = function(){
    var k = this.stiffness,
        d = this.damping,
        l = this.restAngle,
        bodyA = this.bodyA,
        bodyB = this.bodyB,
        x = bodyB.angle - bodyA.angle,
        u = bodyB.angularVelocity - bodyA.angularVelocity;

    var torque = - k * (x - l) - d * u * 0;

    bodyA.angularForce -= torque;
    bodyB.angularForce += torque;
};