diff --git a/maths/free_fall_distance.rb b/maths/free_fall_distance.rb
new file mode 100644
index 0000000..2aee86a
--- /dev/null
+++ b/maths/free_fall_distance.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+# Free-fall helpers based on constant acceleration motion.
+class FreeFallDistance
+  STANDARD_GRAVITY = 9.80665
+
+  class << self
+    # d = 0.5 * g * t^2
+    def distance(time:, gravity: STANDARD_GRAVITY)
+      raise ArgumentError, 'time must be non-negative' if time.negative?
+      raise ArgumentError, 'gravity must be positive' unless gravity.positive?
+
+      0.5 * gravity * time * time
+    end
+
+    # t = sqrt(2d / g)
+    def time(distance:, gravity: STANDARD_GRAVITY)
+      raise ArgumentError, 'distance must be non-negative' if distance.negative?
+      raise ArgumentError, 'gravity must be positive' unless gravity.positive?
+
+      Math.sqrt((2.0 * distance) / gravity)
+    end
+  end
+end
diff --git a/maths/free_fall_distance_test.rb b/maths/free_fall_distance_test.rb
new file mode 100644
index 0000000..ae25015
--- /dev/null
+++ b/maths/free_fall_distance_test.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'minitest/autorun'
+require_relative './free_fall_distance'
+
+class FreeFallDistanceTest < Minitest::Test
+  def test_distance_from_time
+    assert_in_delta 19.6133, FreeFallDistance.distance(time: 2.0), 1E-4
+  end
+
+  def test_time_from_distance
+    assert_in_delta 2.0, FreeFallDistance.time(distance: 19.6133), 1E-4
+  end
+
+  def test_negative_time_raises
+    assert_raises DomainError do
+      FreeFallDistance.distance(time: -1.0)
+    end
+  end
+
+  def test_negative_distance_raises
+    assert_raises DomainError do
+      FreeFallDistance.time(distance: -1.0)
+    end
+  end
+end