``` 001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 ``` ```package algs61; // section 6.1 import stdlib.*; import java.awt.Color; /* *********************************************************************** * Compilation: javac Particle.java * * A particle moving in the unit box with a given position, velocity, * radius, and mass. * *************************************************************************/ public class Particle { private static final double INFINITY = Double.POSITIVE_INFINITY; private double rx, ry; // position private double vx, vy; // velocity private final double radius; // radius private final double mass; // mass private final Color color; // color private int count; // number of collisions so far // create a new particle with given parameters public Particle(double rx, double ry, double vx, double vy, double radius, double mass, Color color) { this.vx = vx; this.vy = vy; this.rx = rx; this.ry = ry; this.radius = radius; this.mass = mass; this.color = color; } // create a random particle in the unit box (overlaps not checked) public Particle() { rx = Math.random(); ry = Math.random(); vx = 0.01 * (Math.random() - 0.5); vy = 0.01 * (Math.random() - 0.5); radius = 0.01; mass = 0.5; //color = Color.BLACK; color = new Color (StdRandom.uniform(200), StdRandom.uniform(200), StdRandom.uniform(200)); } // updates position public void move(double dt) { rx += vx * dt; ry += vy * dt; } // draw the particle public void draw() { StdDraw.setPenColor(color); StdDraw.filledCircle(rx, ry, radius); } // return the number of collisions involving this particle public int count() { return count; } // how long into future until collision between this particle a and b? public double timeToHit(Particle b) { Particle a = this; if (a == b) return INFINITY; double dx = b.rx - a.rx; double dy = b.ry - a.ry; double dvx = b.vx - a.vx; double dvy = b.vy - a.vy; double dvdr = dx*dvx + dy*dvy; if (dvdr > 0) return INFINITY; double dvdv = dvx*dvx + dvy*dvy; double drdr = dx*dx + dy*dy; double sigma = a.radius + b.radius; double d = (dvdr*dvdr) - dvdv * (drdr - sigma*sigma); // if (drdr < sigma*sigma) StdOut.println("overlapping particles"); if (d < 0) return INFINITY; return -(dvdr + Math.sqrt(d)) / dvdv; } // how long into future until this particle collides with a vertical wall? public double timeToHitVerticalWall() { if (vx > 0) return (1.0 - rx - radius) / vx; else if (vx < 0) return (radius - rx) / vx; else return INFINITY; } // how long into future until this particle collides with a horizontal wall? public double timeToHitHorizontalWall() { if (vy > 0) return (1.0 - ry - radius) / vy; else if (vy < 0) return (radius - ry) / vy; else return INFINITY; } // update velocities upon collision between this particle and that particle public void bounceOff(Particle that) { double dx = that.rx - this.rx; double dy = that.ry - this.ry; double dvx = that.vx - this.vx; double dvy = that.vy - this.vy; double dvdr = dx*dvx + dy*dvy; // dv dot dr double dist = this.radius + that.radius; // distance between particle centers at collison // normal force F, and in x and y directions double F = 2 * this.mass * that.mass * dvdr / ((this.mass + that.mass) * dist); double fx = F * dx / dist; double fy = F * dy / dist; // update velocities according to normal force this.vx += fx / this.mass; this.vy += fy / this.mass; that.vx -= fx / that.mass; that.vy -= fy / that.mass; // update collision counts this.count++; that.count++; } // update velocity of this particle upon collision with a vertical wall public void bounceOffVerticalWall() { vx = -vx; count++; } // update velocity of this particle upon collision with a horizontal wall public void bounceOffHorizontalWall() { vy = -vy; count++; } // return kinetic energy associated with this particle public double kineticEnergy() { return 0.5 * mass * (vx*vx + vy*vy); } } ```