diff --git a/docs/physics/spacetime/special-formalism.md b/docs/physics/spacetime/special-formalism.md
index 209ac73..12252be 100644
--- a/docs/physics/spacetime/special-formalism.md
+++ b/docs/physics/spacetime/special-formalism.md
@@ -247,7 +247,7 @@ The four-acceleration can only change the direction of the four-velocity, as the
 A related concept is the **four-force** which in terms of the four-momentum may be defined as
 
 $$
-    \mathbf{f} = d_\tau \mathbf{p} \overset{\mathrm{def}}{=} m d^2_\tau \mathbf{a}.
+    \mathbf{f} = d_\tau \mathbf{p} \overset{\mathrm{def}}{=} m \mathbf{a}.
 $$
 
 In the case that a constant force $F$ in the $x$ direction acts on a body we have that the four-force on that body adheres to