How to Apply Linear Impulse to Center of Mass in Planck.js?

Applying a linear impulse directly to the center of mass of a planck.js body changes its linear velocity without inducing any angular rotation (torque). This article provides a quick overview of how to calculate a body’s center of mass, invoke the proper applyLinearImpulse method, and avoid unintended rotational forces in your 2D physics simulations.

Understanding the Method

In planck.js (a JavaScript port of the Box2D physics engine), the Body object provides a built-in method called applyLinearImpulse. To apply an impulse specifically to the center of mass, you must pass the impulse vector and the body’s current world center position as arguments.

The method signature generally looks like this:

body.applyLinearImpulse(impulse, point, wake);

• impulse: A Vec2 object representing the direction and magnitude of the force applied over time (in Newton-seconds).
• point: A Vec2 object representing the world coordinates where the impulse is applied. To avoid rotation, this must be the center of mass.
• wake: A boolean (usually true) indicating whether to wake up the body if it is currently sleeping.

Step-by-Step Implementation

1. Get the Center of Mass

You can retrieve the precise world coordinates of a body’s center of mass using the getWorldCenter() method. This ensures that even if the body has moved or rotated, you are targeting the exact point of balance.

const centerOfMass = body.getWorldCenter();

2. Define the Impulse Vector

Create a 2D vector that defines how much force you want to apply and in what direction.

const Vec2 = planck.Vec2;
const impulse = Vec2(10.0, 0.0); // Applies a force of 10 units to the right

3. Apply the Impulse

Pass both the impulse vector and the center of mass vector into the method.

body.applyLinearImpulse(impulse, centerOfMass, true);

Why the Application Point Matters

When an impulse is applied to any point other than the center of mass, it creates a turning moment (torque) based on the distance between the application point and the center of mass. By explicitly passing body.getWorldCenter() as the application point, the lever arm length is effectively zero. This guarantees that the object will move in a straight line according to the impulse vector without spinning.