diff --git a/asteroids/main.lua b/asteroids/main.lua
index a0ad3ee..0bc5df9 100644
--- a/asteroids/main.lua
+++ b/asteroids/main.lua
@@ -1,4 +1,5 @@
 local love = require("love")
+local phys = require("phys")
 
 local function initGame()
     return {
@@ -68,7 +69,8 @@ local function initState()
             speed = {
                 x = 0,
                 y = 0,
-            }
+            },
+            radius = game.ship.radius,
         },
         bullets = {},
         asteroids = asteroids,
@@ -88,17 +90,11 @@ function love.load()
     reset()
 end
 
--- updates the actor's location by applying the passage of time
--- TODO ship
--- TODO spin
+--  updates the actor's location by applying the passage of time within the game world
 local function update(actor, dt)
-    local dv = actor.speed * dt
-    actor.x = (actor.x + math.cos(actor.angle) * dv) % game.width
-    actor.y = (actor.y + math.sin(actor.angle) * dv) % game.height
-end
-
-local function intersects(actor1, actor2)
-    return (actor1.x - actor2.x) ^ 2 + (actor1.y - actor2.y) ^ 2 <= (actor1.radius + actor2.radius) ^ 2
+    phys.update(actor, dt)
+    actor.x = actor.x % game.width
+    actor.y = actor.y % game.height
 end
 
 function love.update(dt)
@@ -113,10 +109,6 @@ function love.update(dt)
     state.ship.x = (state.ship.x + state.ship.speed.x * dt) % game.width
     state.ship.y = (state.ship.y + state.ship.speed.y * dt) % game.height
 
-    local function areCirclesIntersecting(aX, aY, aRadius, bX, bY, bRadius)
-        return (aX - bX) ^ 2 + (aY - bY) ^ 2 <= (aRadius + bRadius) ^ 2
-    end
-
     for bulletIndex = #state.bullets, 1, -1 do
         local bullet = state.bullets[bulletIndex]
         bullet.timeLeft = bullet.timeLeft - dt
@@ -127,7 +119,7 @@ function love.update(dt)
         end
         for asteroidIndex = #state.asteroids, 1, -1 do
             local asteroid = state.asteroids[asteroidIndex]
-            if intersects(bullet, asteroid) then
+            if phys.intersects(bullet, asteroid) then
                 table.remove(state.bullets, bulletIndex)
                 if asteroid.stage > 1 then
                     local angle1 = love.math.random() * (2 * math.pi)
@@ -160,7 +152,7 @@ function love.update(dt)
     for _, asteroid in ipairs(state.asteroids) do
         update(asteroid, dt)
 
-        if areCirclesIntersecting(state.ship.x, state.ship.y, game.ship.radius, asteroid.x, asteroid.y, asteroid.radius) then
+        if phys.intersects(state.ship, asteroid) then
             reset()
             break
         end
diff --git a/asteroids/phys.lua b/asteroids/phys.lua
new file mode 100644
index 0000000..edc5920
--- /dev/null
+++ b/asteroids/phys.lua
@@ -0,0 +1,18 @@
+local phys = {
+    tau = 2 * math.pi
+}
+
+-- returns true if the two actors intersect
+function phys.intersects(actor1, actor2)
+    return (actor1.x - actor2.x) ^ 2 + (actor1.y - actor2.y) ^ 2 <= (actor1.radius + actor2.radius) ^ 2
+end
+
+-- updates the actor's location by applying the passage of time
+-- TODO spin
+function phys.update(actor, dt)
+    local dv = actor.speed * dt
+    actor.x = actor.x + math.cos(actor.angle) * dv
+    actor.y = actor.y + math.sin(actor.angle) * dv
+end
+
+return phys