[FEAT] Add general purpose Projectile Motion algorithm (Physics) (#6928)

[FEAT] Add general purpose Projectile Motion algorithm

Co-authored-by: Priyanshu1303d <[email protected]>
Co-authored-by: Deniz Altunkapan <[email protected]>

Author: 3 people

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

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+package com.thealgorithms.physics;
+
+/**
+ *
+ * This implementation calculates the flight path of a projectile launched from any INITIAL HEIGHT.
+ * It is a more flexible version of the ground-to-ground model.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Projectile_motion">Wikipedia - Projectile Motion</a>
+ * @author [Priyanshu Kumar Singh](https://github.com/Priyanshu1303d)
+ */
+public final class ProjectileMotion {
+
+    private ProjectileMotion() {
+    }
+
+    /** Standard Earth gravity constant*/
+    private static final double GRAVITY = 9.80665;
+
+    /**
+     * A simple container for the results of a projectile motion calculation.
+     */
+    public static final class Result {
+        private final double timeOfFlight;
+        private final double horizontalRange;
+        private final double maxHeight;
+
+        public Result(double timeOfFlight, double horizontalRange, double maxHeight) {
+            this.timeOfFlight = timeOfFlight;
+            this.horizontalRange = horizontalRange;
+            this.maxHeight = maxHeight;
+        }
+
+        /** @return The total time the projectile is in the air (seconds). */
+        public double getTimeOfFlight() {
+            return timeOfFlight;
+        }
+
+        /** @return The total horizontal distance traveled (meters). */
+        public double getHorizontalRange() {
+            return horizontalRange;
+        }
+
+        /** @return The maximum vertical height from the ground (meters). */
+        public double getMaxHeight() {
+            return maxHeight;
+        }
+    }
+
+    /**
+     * Calculates projectile trajectory using standard Earth gravity.
+     *
+     * @param initialVelocity Initial speed of the projectile (m/s).
+     * @param launchAngleDegrees Launch angle from the horizontal (degrees).
+     * @param initialHeight Starting height of the projectile (m).
+     * @return A {@link Result} object with the trajectory data.
+     */
+    public static Result calculateTrajectory(double initialVelocity, double launchAngleDegrees, double initialHeight) {
+        return calculateTrajectory(initialVelocity, launchAngleDegrees, initialHeight, GRAVITY);
+    }
+
+    /**
+     * Calculates projectile trajectory with a custom gravity value.
+     *
+     * @param initialVelocity Initial speed (m/s). Must be non-negative.
+     * @param launchAngleDegrees Launch angle (degrees).
+     * @param initialHeight Starting height (m). Must be non-negative.
+     * @param gravity Acceleration due to gravity (m/s^2). Must be positive.
+     * @return A {@link Result} object with the trajectory data.
+     */
+    public static Result calculateTrajectory(double initialVelocity, double launchAngleDegrees, double initialHeight, double gravity) {
+        if (initialVelocity < 0 || initialHeight < 0 || gravity <= 0) {
+            throw new IllegalArgumentException("Velocity, height, and gravity must be non-negative, and gravity must be positive.");
+        }
+
+        double launchAngleRadians = Math.toRadians(launchAngleDegrees);
+        double initialVerticalVelocity = initialVelocity * Math.sin(launchAngleRadians); // Initial vertical velocity
+        double initialHorizontalVelocity = initialVelocity * Math.cos(launchAngleRadians); // Initial horizontal velocity
+
+        // Correctly calculate total time of flight using the quadratic formula for vertical motion.
+        // y(t) = y0 + initialVerticalVelocity*t - 0.5*g*t^2. We solve for t when y(t) = 0.
+        double totalTimeOfFlight = (initialVerticalVelocity + Math.sqrt(initialVerticalVelocity * initialVerticalVelocity + 2 * gravity * initialHeight)) / gravity;
+
+        // Calculate max height. If launched downwards, max height is the initial height.
+        double maxHeight;
+        if (initialVerticalVelocity > 0) {
+            double heightGained = initialVerticalVelocity * initialVerticalVelocity / (2 * gravity);
+            maxHeight = initialHeight + heightGained;
+        } else {
+            maxHeight = initialHeight;
+        }
+
+        double horizontalRange = initialHorizontalVelocity * totalTimeOfFlight;
+
+        return new Result(totalTimeOfFlight, horizontalRange, maxHeight);
+    }
+}

src/test/java/com/thealgorithms/physics/ProjectileMotionTest.java

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+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test class for the general-purpose ProjectileMotion calculator.
+ *
+ */
+final class ProjectileMotionTest {
+
+    private static final double DELTA = 1e-4; // Tolerance for comparing double values
+
+    @Test
+    @DisplayName("Test ground-to-ground launch (initial height is zero)")
+    void testGroundToGroundLaunch() {
+        ProjectileMotion.Result result = ProjectileMotion.calculateTrajectory(50, 30, 0);
+        assertEquals(5.0986, result.getTimeOfFlight(), DELTA);
+        assertEquals(220.7750, result.getHorizontalRange(), DELTA);
+        assertEquals(31.8661, result.getMaxHeight(), DELTA);
+    }
+
+    @Test
+    @DisplayName("Test launch from an elevated position")
+    void testElevatedLaunch() {
+        ProjectileMotion.Result result = ProjectileMotion.calculateTrajectory(30, 45, 100);
+        assertEquals(7.1705, result.getTimeOfFlight(), DELTA);
+        assertEquals(152.1091, result.getHorizontalRange(), DELTA);
+        assertEquals(122.9436, result.getMaxHeight(), DELTA); // Final corrected value
+    }
+
+    @Test
+    @DisplayName("Test launch straight up (90 degrees)")
+    void testVerticalLaunch() {
+        ProjectileMotion.Result result = ProjectileMotion.calculateTrajectory(40, 90, 20);
+        assertEquals(8.6303, result.getTimeOfFlight(), DELTA);
+        assertEquals(0.0, result.getHorizontalRange(), DELTA);
+        assertEquals(101.5773, result.getMaxHeight(), DELTA);
+    }
+
+    @Test
+    @DisplayName("Test horizontal launch from a height (0 degrees)")
+    void testHorizontalLaunch() {
+        ProjectileMotion.Result result = ProjectileMotion.calculateTrajectory(25, 0, 80);
+        assertEquals(4.0392, result.getTimeOfFlight(), DELTA);
+        assertEquals(100.9809, result.getHorizontalRange(), DELTA);
+        assertEquals(80.0, result.getMaxHeight(), DELTA);
+    }
+
+    @Test
+    @DisplayName("Test downward launch from a height (negative angle)")
+    void testDownwardLaunchFromHeight() {
+        ProjectileMotion.Result result = ProjectileMotion.calculateTrajectory(20, -30, 100);
+        assertEquals(3.6100, result.getTimeOfFlight(), DELTA);
+        assertEquals(62.5268, result.getHorizontalRange(), DELTA);
+        assertEquals(100.0, result.getMaxHeight(), DELTA);
+    }
+
+    @Test
+    @DisplayName("Test invalid arguments throw an exception")
+    void testInvalidInputs() {
+        assertThrows(IllegalArgumentException.class, () -> ProjectileMotion.calculateTrajectory(-10, 45, 100));
+        assertThrows(IllegalArgumentException.class, () -> ProjectileMotion.calculateTrajectory(10, 45, -100));
+        assertThrows(IllegalArgumentException.class, () -> ProjectileMotion.calculateTrajectory(10, 45, 100, 0));
+    }
+}