diff --git a/libraries/particles/src/Particle.cpp b/libraries/particles/src/Particle.cpp
index edca3a1fc0..346760b115 100644
--- a/libraries/particles/src/Particle.cpp
+++ b/libraries/particles/src/Particle.cpp
@@ -54,6 +54,7 @@ void Particle::init(glm::vec3 position, float radius, rgbColor color, glm::vec3
     
     _position = position;
     _radius = radius;
+    _mass = 1.0f;
     memcpy(_color, color, sizeof(_color));
     _velocity = velocity;
     _damping = damping;
@@ -63,6 +64,11 @@ void Particle::init(glm::vec3 position, float radius, rgbColor color, glm::vec3
     _shouldDie = false;
 }
 
+void Particle::setMass(float value) {
+    if (value > 0.0f) {
+        _mass = value;
+    }
+}
 
 bool Particle::appendParticleData(OctreePacketData* packetData) const {
 
diff --git a/libraries/particles/src/Particle.h b/libraries/particles/src/Particle.h
index 9c92d0ec48..5e80234660 100644
--- a/libraries/particles/src/Particle.h
+++ b/libraries/particles/src/Particle.h
@@ -67,6 +67,7 @@ public:
     const rgbColor& getColor() const { return _color; }
     xColor getXColor() const { xColor color = { _color[RED_INDEX], _color[GREEN_INDEX], _color[BLUE_INDEX] }; return color; }
     float getRadius() const { return _radius; }
+    float getMass() const { return _mass; }
     const glm::vec3& getVelocity() const { return _velocity; }
     const glm::vec3& getGravity() const { return _gravity; }
     bool getInHand() const { return _inHand; }
@@ -96,6 +97,7 @@ public:
             _color[BLUE_INDEX] = value.blue; 
     }
     void setRadius(float value) { _radius = value; }
+    void setMass(float value); 
     void setGravity(const glm::vec3& value) { _gravity = value; }
     void setInHand(bool inHand) { _inHand = inHand; }
     void setDamping(float value) { _damping = value; }
@@ -146,6 +148,7 @@ protected:
     glm::vec3 _position;
     rgbColor _color;
     float _radius;
+    float _mass;
     glm::vec3 _velocity;
     uint32_t _id;
     static uint32_t _nextID;
diff --git a/libraries/particles/src/ParticleCollisionSystem.cpp b/libraries/particles/src/ParticleCollisionSystem.cpp
index 1e1f3955fe..0389dddb66 100644
--- a/libraries/particles/src/ParticleCollisionSystem.cpp
+++ b/libraries/particles/src/ParticleCollisionSystem.cpp
@@ -87,41 +87,50 @@ void ParticleCollisionSystem::updateCollisionWithVoxels(Particle* particle) {
     }
 }
 
-void ParticleCollisionSystem::updateCollisionWithParticles(Particle* particle) {
-    glm::vec3 center = particle->getPosition() * static_cast<float>(TREE_SCALE);
-    float radius = particle->getRadius() * static_cast<float>(TREE_SCALE);
-    const float ELASTICITY = 1.4f;
+void ParticleCollisionSystem::updateCollisionWithParticles(Particle* particleA) {
+    glm::vec3 center = particleA->getPosition() * (float)TREE_SCALE;
+    float radius = particleA->getRadius() * (float)TREE_SCALE;
+    const float ELASTICITY = 0.4f;
     const float DAMPING = 0.0f;
     const float COLLISION_FREQUENCY = 0.5f;
     glm::vec3 penetration;
-    Particle* penetratedParticle;
-    if (_particles->findSpherePenetration(center, radius, penetration, (void**)&penetratedParticle)) {
+    Particle* particleB;
+    if (_particles->findSpherePenetration(center, radius, penetration, (void**)&particleB)) {
+        // NOTE: 'penetration' is the depth that 'particleA' overlaps 'particleB'.
+        // That is, it points from A into B.
     
-        // let the particles run their collision scripts if they have them
-        particle->collisionWithParticle(penetratedParticle);
-        penetratedParticle->collisionWithParticle(particle);
+        // Even if the particles overlap... when the particles are already moving appart
+        // we don't want to count this as a collision.
+        glm::vec3 relativeVelocity = particleA->getVelocity() - particleB->getVelocity();
+        if (glm::dot(relativeVelocity, penetration) > 0.0f) {
+            particleA->collisionWithParticle(particleB);
+            particleB->collisionWithParticle(particleA);
 
-        penetration /= static_cast<float>(TREE_SCALE);
-        updateCollisionSound(particle, penetration, COLLISION_FREQUENCY);
-        
-        // apply a hard collision to both particles of half the penetration each
-        
-        float particleShare, penetratedParticleShare;
-        if (particle->getInHand() && penetratedParticle->getInHand()) {
-            particleShare = 0.5f;
-            penetratedParticleShare = -0.5f;
-        } else if (particle->getInHand()) {
-            particleShare = 0.f;
-            penetratedParticleShare = -1.f;
-        } else if (penetratedParticle->getInHand()) {
-            particleShare = -1.f;
-            penetratedParticleShare = 0.f;
-        } else {
-            particleShare = 0.5f;
-            penetratedParticleShare = -0.5f;
+            glm::vec3 axis = glm::normalize(penetration);
+            glm::vec3 axialVelocity = glm::dot(relativeVelocity, axis) * axis;
+
+            // particles that are in hand are assigned an ureasonably large mass for collisions
+            // which effectively makes them immovable but allows the other ball to reflect correctly.
+            const float MAX_MASS = 1.0e6f;
+            float massA = (particleA->getInHand()) ? MAX_MASS : particleA->getMass();
+            float massB = (particleB->getInHand()) ? MAX_MASS : particleB->getMass();
+            float totalMass = massA + massB;
+    
+            particleA->setVelocity(particleA->getVelocity() - axialVelocity * (2.0f * massB / totalMass));
+
+            ParticleEditHandle particleEditHandle(_packetSender, _particles, particleA->getID());
+            particleEditHandle.updateParticle(particleA->getPosition(), particleA->getRadius(), particleA->getXColor(), particleA->getVelocity(),
+                           particleA->getGravity(), particleA->getDamping(), particleA->getInHand(), particleA->getScript());
+
+            particleB->setVelocity(particleB->getVelocity() + axialVelocity * (2.0f * massA / totalMass));
+
+            ParticleEditHandle penetratedparticleEditHandle(_packetSender, _particles, particleB->getID());
+            penetratedparticleEditHandle.updateParticle(particleB->getPosition(), particleB->getRadius(), particleB->getXColor(), particleB->getVelocity(),
+                           particleB->getGravity(), particleB->getDamping(), particleB->getInHand(), particleB->getScript());
+
+            penetration /= (float)TREE_SCALE;
+            updateCollisionSound(particleA, penetration, COLLISION_FREQUENCY);
         }
-        applyHardCollision(particle, penetration * particleShare, ELASTICITY, DAMPING);
-        applyHardCollision(penetratedParticle, penetration * penetratedParticleShare, ELASTICITY, DAMPING);
     }
 }