Calculate Ammo.js Hinge Breaking Impulse

This article explains how to calculate and implement the breaking impulse threshold for a hinge constraint in the Ammo.js physics engine. You will learn the core physics formula used to determine this threshold based on maximum force and simulation time steps, as well as how to apply this calculation within your JavaScript physics loops.

In Ammo.js (a direct port of the Bullet physics engine), constraints like hinges do not possess a built-in “break” property that automatically destroys them. Instead, you must calculate a threshold value—the breaking impulse—and check it against the impulse applied by the physics solver during each simulation tick. If the applied impulse exceeds your calculated threshold, you manually destroy the constraint.

The Breaking Impulse Formula

To find the required breaking impulse (\(J_{break}\)), you use the relationship between impulse, force, and time. Impulse is the integral of force over time. For a constant or average maximum force applied over a single physics time step, the formula is:

\[J_{break} = F_{max} \times \Delta t\]

For rotational constraints where torque is the limiting factor:

\[J_{break} = \tau_{max} \times \Delta t\]

Where: * \(J_{break}\) is the breaking impulse threshold (measured in Newton-seconds, \(N \cdot s\)). * \(F_{max}\) is the maximum linear force (Newtons, \(N\)) the hinge can withstand before snapping. * \(\tau_{max}\) is the maximum torque (Newton-meters, \(N \cdot m\)) the hinge can withstand. * \(\Delta t\) is the fixed time step of your physics simulation. In Ammo.js, this is typically \(1/60\) of a second (\(0.01667\) seconds) or whatever value you pass to dynamicsWorld.stepSimulation().

Practical Implementation in Ammo.js

To apply this formula in your project, follow these steps:

  1. Calculate the Threshold: If you want a hinge to break when subjected to a force greater than 500 Newtons, and your physics simulation runs at 60Hz (\(\Delta t = 0.01667\)s): \[J_{break} = 500 \times 0.01667 \approx 8.33 \text{ N}\cdot\text{s}\]

  2. Retrieve the Applied Impulse: During your game loop (ideally right after dynamicsWorld.stepSimulation()), query the hinge constraint for the impulse applied during the last frame using the getAppliedImpulse() method.

  3. Evaluate and Break: Compare the absolute value of the applied impulse to your calculated threshold. If it exceeds the threshold, remove the constraint from the physics world.

// Example check inside the physics update loop
const timeStep = 1.0 / 60.0;
const maxForce = 500.0; // Newtons
const breakingImpulse = maxForce * timeStep;

function updatePhysics() {
    dynamicsWorld.stepSimulation(timeStep, 10);

    // Get the impulse applied to resolve the hinge constraint on the last step
    const appliedImpulse = hingeConstraint.getAppliedImpulse();

    // Check if the impulse exceeds the breaking threshold
    if (Math.abs(appliedImpulse) > breakingImpulse) {
        // Remove the constraint to break the hinge
        dynamicsWorld.removeConstraint(hingeConstraint);
        ammo.destroy(hingeConstraint);
        console.log("Hinge broke due to excessive force!");
    }
}