4 files changed, 82 insertions, 0 deletions
diff --git a/physics/src/force/drag.rs b/physics/src/force/drag.rs
new file mode 100644
index 0000000..c54dc40
--- /dev/null
+++ b/physics/src/force/drag.rs
@@ -0,0 +1,15 @@
+use crate::algebra::Scalar;
+
+use super::Force;
+
+pub struct Drag {
+    pub coefficient: Scalar,
+}
+
+impl Force for Drag {
+    fn apply(&self, particles: &mut Vec<crate::particle_system::Particle>) {
+        for particle in particles {
+            particle.apply_force(-self.coefficient * particle.velocity);
+        }
+    }
+}
diff --git a/physics/src/force/gravity.rs b/physics/src/force/gravity.rs
new file mode 100644
index 0000000..dd600a4
--- /dev/null
+++ b/physics/src/force/gravity.rs
@@ -0,0 +1,15 @@
+use crate::algebra::Vector;
+
+use super::Force;
+
+pub struct Gravity {
+    pub vector: Vector,
+}
+
+impl Force for Gravity {
+    fn apply(&self, particles: &mut Vec<crate::particle_system::Particle>) {
+        for particle in particles {
+            particle.apply_force(self.vector * particle.mass);
+        }
+    }
+}
diff --git a/physics/src/force/mod.rs b/physics/src/force/mod.rs
new file mode 100644
index 0000000..ce10f9f
--- /dev/null
+++ b/physics/src/force/mod.rs
@@ -0,0 +1,24 @@
+use crate::particle_system::{Particle, ParticleSystem};
+
+pub mod gravity;
+pub mod drag;
+pub mod spring;
+
+pub trait Force {
+    fn apply(&self, particles: &mut Vec<Particle>);
+}
+
+impl ParticleSystem {
+    fn reset_forces(&mut self) {
+        for particle in &mut self.particles {
+            particle.reset_force();
+        }
+    }
+    pub fn apply_forces(&mut self) {
+        self.reset_forces();
+
+        for force in &self.forces {
+            force.apply(&mut self.particles)
+        }
+    }
+}
diff --git a/physics/src/force/spring.rs b/physics/src/force/spring.rs
new file mode 100644
index 0000000..6c2cf67
--- /dev/null
+++ b/physics/src/force/spring.rs
@@ -0,0 +1,28 @@
+use crate::algebra::Scalar;
+
+use super::Force;
+
+pub struct Spring {
+    pub particle_ids: [usize; 2],
+    pub rest_length: Scalar,
+    pub spring_constant: Scalar,
+    pub damping_constant: Scalar,
+}
+
+impl Force for Spring {
+    fn apply(&self, particles: &mut Vec<crate::particle_system::Particle>) {
+        let a = &particles[self.particle_ids[0]];
+        let b = &particles[self.particle_ids[1]];
+        let i = a.position - b.position;
+        let i_dot = a.velocity - b.velocity;
+        let i_norm = i.norm();
+
+        let force = -(self.spring_constant * (i_norm - self.rest_length)
+        + (self.damping_constant * i.dot(&i_dot) / i_norm))
+        * i
+        / i_norm;
+
+        particles[self.particle_ids[0]].apply_force(force);
+        particles[self.particle_ids[1]].apply_force(-force);
+    }
+}