The speed the two shapes were moving towards or away from each other along the manifold normal at the start of the simulation step, in meters per second. A negative value means the shapes were approaching. This is always measured when the contact is bouncy (a bounciness above zero) or either shape has hit events enabled. Otherwise it is usually zero but can hold a measured value, so only rely on it under those conditions.

public readonly float normalVelocity { get; }