Update index.md

content/blog/1d-collisions/index.md

@@ -1,3 +1,10 @@
+
+## 3.
+
+Firstly, it is essential to comprehend how the velocity of the bodies after collision is calculated. We will solve the problem in the elastic collision model (one-dimensional Newtonian).
+
+# 3.1
+
 $$
 m_{1}\vec{v}_{1} + m_{2}\vec{v}_{2} = m_{1}\vec{v'}_{1} + m_{2}\vec{v'}_{2}
 $$