Flame: Forces, impulses & linear velocity

We know that Forge2D has forces that, when applied to the bodies, will move and create a realistic simulation.

In this article, we are going to create a Ball class that will receive the position on the X-axis, then we are going to place three balls on the floor and apply different kinds of forces.

class Ball extends BodyComponent {
  final double positionX;

  Ball(this.positionX);

  @override
  Body createBody() {
    final bodyDef = BodyDef(
      position: Vector2(positionX, worldSize.y - 1),
      type: BodyType.dynamic,
    );

    final shape = CircleShape()..radius = .35;
    final fixtureDef = FixtureDef(shape)
      ..density = 5
      ..friction = .5
      ..restitution = .5;
    return world.createBody(bodyDef)..createFixture(fixtureDef);
  }
}

Let's add the balls to the game, but because we want to keep track of the balls so we can apply a force each time the screen is clicked, we will store the ball objects in a list:

class GameLesson04 extends MyGame with TapDetector {
  final balls = [Ball(2), Ball(6), Ball(10)];
  final speed = Vector2(0, -9);

  @override
  Future<void> onLoad() async {
    super.onLoad();

    add(Floor());
    addAll(balls);
  }

  @override
  void onTapDown(TapDownInfo info) {
    super.onTapDown(info);
    balls[0].body.applyForce(speed);
    balls[1].body.applyLinearImpulse(speed);
    balls[2].body.linearVelocity = speed;
  }
}

The list balls contains the three balls placed horizontally on the x positions 2, 6, and 10.

The speed vector is the value we will use to apply a force to the body. In this case, the body will only move vertically because the Y-axis value is not zero, and the value on the X-axis is zero, so the body will not move horizontally.

We are using TapDetector and overriding the function onTapDown(), so every time there is a click on the screen, we will interact with the bodies.

After running the code, it will look like this:

Balls resting on the floor

Applying forces

To interact with the balls, we tap or click the screen, and then the following code will be executed:

balls[0].body.applyForce(speed);
balls[1].body.applyLinearImpulse(speed);
balls[2].body.linearVelocity = speed;

And the result will look like this:

Balls reacting to forces

As you have noticed, the movement of the ball is different. Each ball is calling a different function. Let's check what each of them is doing:

applyForce: To the ball on the left, we are applying a force. Forces change the velocity of a body gradually over time. For example, an accelerating car will start moving slowly, and over time it will reach the desired velocity.

applyLinearImpulse: To the ball in the middle, we apply a linear impulse. Impulses make immediate changes to the body’s velocity. For example, kicking a soccer ball will immediately change the ball's velocity and make it move far away, but what if you kick a big rock? Probably it will not move very far.

linearVelocity: To the ball on the right, we set the velocity, so no matter the body's size and mass, we just "magically" set the desired velocity.