Added a gravity system.

app/src/main.rs

@@ -2,12 +2,13 @@ use bevy::color::palettes::basic::RED;
 use bevy::log;
 use bevy::prelude::*;                   // Core Bevy types
 
+const GRAVITY: f32 = -20.0;
 
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)
-        .add_systems(Update, velocity_timestep)
+        .add_systems(Update, (apply_gravity, velocity_timestep).chain())
         .run();
 }
 
@@ -17,10 +18,17 @@ fn setup(mut commands: Commands, mut meshes: ResMut<Assets<Mesh>>, mut materials
 
     let shape =meshes.add(Circle::new(50.0));
 
-    commands.spawn((Mesh2d(shape.clone()), MeshMaterial2d(materials.add(Color::from(RED))), Transform::from_xyz(100.0, 0.0, 0.0), Velocity(vec3(0.0, 100.0, 0.0))));
+    commands.spawn((Mesh2d(shape.clone()), MeshMaterial2d(materials.add(Color::from(RED))), Transform::from_xyz(100.0, 0.0, 0.0), Velocity(vec3(
+        0.0,
+        100.0,
+        0.0
+    ))));
 
-    commands.spawn((Mesh2d(shape.clone()), MeshMaterial2d(materials.add(Color::from(RED))), Transform::from_xyz(0.0, 0.0, 0.0), Velocity(vec3(0.0, 100.0, 0.0))));
-}
+    commands.spawn((Mesh2d(shape.clone()), MeshMaterial2d(materials.add(Color::from(RED))), Transform::from_xyz(300.0, 0.0, 0.0), Velocity(vec3(
+        0.0,
+        50.0,
+        0.0
+    ))));}
 
 #[derive(Component)]
 struct Velocity(Vec3);
@@ -29,4 +37,10 @@ fn velocity_timestep(time: Res<Time>, mut query: Query<(&Velocity, &mut Transfor
     for (velocity, mut transform) in query.iter_mut() {
         transform.translation += velocity.0 * time.delta_secs();
     }
+}
+
+fn apply_gravity(time: Res<Time>, mut query: Query<&mut Velocity>) {
+    for mut velocity in query.iter_mut() {
+        velocity.0.y += GRAVITY * time.delta_secs();
+    }
 }