Adding ElasticCollision2D (#6802)

* Adding ElasticCollision2D

* Fixing build issues.

src/main/java/com/thealgorithms/physics/ElasticCollision2D.java

@@ -0,0 +1,73 @@
+package com.thealgorithms.physics;
+
+/**
+ * 2D Elastic collision between two circular bodies
+ * Based on principles of conservation of momentum and kinetic energy.
+ *
+ * @author [Yash Rajput](https://github.com/the-yash-rajput)
+ */
+public final class ElasticCollision2D {
+
+    private ElasticCollision2D() {
+        throw new AssertionError("No instances. Utility class");
+    }
+
+    public static class Body {
+        public double x;
+        public double y;
+        public double vx;
+        public double vy;
+        public double mass;
+        public double radius;
+
+        public Body(double x, double y, double vx, double vy, double mass, double radius) {
+            this.x = x;
+            this.y = y;
+            this.vx = vx;
+            this.vy = vy;
+            this.mass = mass;
+            this.radius = radius;
+        }
+    }
+
+    /**
+     * Resolve instantaneous elastic collision between two circular bodies.
+     *
+     * @param a first body
+     * @param b second body
+     */
+    public static void resolveCollision(Body a, Body b) {
+        double dx = b.x - a.x;
+        double dy = b.y - a.y;
+        double dist = Math.hypot(dx, dy);
+
+        if (dist == 0) {
+            return; // overlapping
+        }
+
+        double nx = dx / dist;
+        double ny = dy / dist;
+
+        // relative velocity along normal
+        double rv = (b.vx - a.vx) * nx + (b.vy - a.vy) * ny;
+
+        if (rv > 0) {
+            return; // moving apart
+        }
+
+        // impulse with masses
+        double m1 = a.mass;
+        double m2 = b.mass;
+
+        double j = -(1 + 1.0) * rv / (1.0 / m1 + 1.0 / m2);
+
+        // impulse vector
+        double impulseX = j * nx;
+        double impulseY = j * ny;
+
+        a.vx -= impulseX / m1;
+        a.vy -= impulseY / m1;
+        b.vx += impulseX / m2;
+        b.vy += impulseY / m2;
+    }
+}