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rigid_body_forces_and_impulses

In addition to gravity, it is possible to add custom forces (or torques) or apply impulses (or torque impulses) to dynamic rigid-bodies in order to make them move in specific ways. Forces affect the rigid-body's acceleration whereas impulses affect the rigid-body's velocity. They are both based on the familiar equations:

  • Forces: the acceleration change is equal to the force divided by the mass: Δa=m−1f\Delta{}a = m^{-1}f
  • Impulses: the velocity change is equal to the impulse divided by the mass: Δv=m−1i\Delta{}v = m^{-1}i

Forces can be added, and impulses can be applied, to a rigid-body after it has been created. Added forces are persistent across simulation steps, and can be cleared manually.

// The `true` argument makes sure the rigid-body is awake.
rigidBody.resetForces(true);  // Reset the forces to zero.
rigidBody.resetTorques(true); // Reset the torques to zero.
rigidBody.addForce({ x: 0.0, y: 1000.0 }, true);
rigidBody.addTorque(100.0, true);
rigidBody.addForceAtPoint({ x: 0.0, y: 1000.0 }, { x: 1.0, y: 2.0 }, true);

rigidBody.applyImpulse({ x: 0.0, y: 1000.0 }, true);
rigidBody.applyTorqueImpulse(100.0, true);
rigidBody.applyImpulseAtPoint({ x: 0.0, y: 1000.0 }, { x: 1.0, y: 2.0 }, true);
info

Keep in mind that a dynamic rigid-body with a zero mass won't be affected by a linear force/impulse, and a rigid-body with a zero angular inertia won't be affected by torques/torque impulses. So if your force doesn't appear to do anything, make sure that:

  1. The rigid-body is dynamic.
  2. It is strong enough to make the rigid-body move (try a very large value and see if it does something).
  3. The rigid-body has a non-zero mass or angular inertia either because they were set explicitly, or because they were computed automatically from colliders with non-zero densities.