diff --git a/common/systems/tests/phys/basic.rs b/common/systems/tests/phys/basic.rs
index 322bf7d063..b9ec33ee4c 100644
--- a/common/systems/tests/phys/basic.rs
+++ b/common/systems/tests/phys/basic.rs
@@ -340,16 +340,16 @@ fn physics_theory() -> Result<(), Box<dyn Error>> {
         //physics
         //let acc2 = (vel - old_vel) / dt;
         //let distance =  0.5 * vel * dt - 0.5 * acc2 * dt * dt;
+        let distance_last = mass / c * ( ( ( old_vel * (c/acc / mass).sqrt()).atanh() ).cosh() ).ln();
         let distance = mass / c * ( ( ( old_vel * (c/acc / mass).sqrt()).atanh() + dt* (c * acc / mass).sqrt()).cosh() ).ln();
-        let distance2 = mass / c * ( ( ( vel * (c/acc / mass).sqrt()).atanh() + dt* (c * acc / mass).sqrt()).cosh() ).ln();
-        let diff = distance2 - distance;
+        let diff = distance - distance_last;
 
         let pos = pos + diff;
 
 
 
         //if ((i+1) as f64 *dt * 10.0).round() as i64 % 2 == 0 {
-            println!("[{:0>2.1}]:    move_dir: {:0>1.1},    acc: {:0>4.4},    vel: {:0>4.4},    dist: {:0>7.4},    dist: {:0>7.4},    pos: {:0>7.4},    c: {:0>4.4}", (i+1) as f64 *dt, move_dir, acc, vel, distance, distance2, pos, c);
+            println!("[{:0>2.1}]:    move_dir: {:0>1.1},    acc: {:0>4.4},    vel: {:0>4.4},    dist: {:0>7.4},    dist: {:0>7.4},    pos: {:0>7.4},    c: {:0>4.4}", (i+1) as f64 *dt, move_dir, acc, vel, distance_last, distance, pos, c);
         //}
         (acc, vel, pos)
     };