 -= accel;
        if(a === 3) this.pvy += accel;

        // handle boundary conditions and bounce
        if(this.ppx < this.rad) {
          this.pvx *= -0.5; // bounce!
          this.ppx = this.rad;
        }
        if(this.ppx > 1 - this.rad) {
          this.pvx *= -0.5;
          this.ppx = 1 - this.rad;
        }
        if(this.ppy < this.rad) {
          this.pvy *= -0.5; // bounce!
          this.ppy = this.rad;
        }
        if(this.ppy > 1 - this.rad) {
          this.pvy *= -0.5;
       