fixed timestep accumulator

private float accumulator = 0;

// when setting up the world
world.setAutoClearForces(false);

@Override
public void update(float delta) {
    float step = 1 / 60f; // the duration of each timestep
    accumulator += delta;

    while(accumulator - step >= 0) {
        accumulator -= step;
        world.step(step, 8, 3);
    }

    world.clearForces();
}