The question "when is the speed zero?" is a bit tricky because it depends on what you're talking about. Here's why:

1. In everyday language:

* We often use "speed" to mean how fast something is moving. In this case, speed is zero when something is not moving at all. It's stationary, still, or at rest.

2. In Physics:

* In physics, speed is a scalar quantity, meaning it only describes how fast something is moving, not its direction.

* Velocity is a vector quantity and describes both speed and direction.

* Therefore, speed can be zero even when something is moving if the object is changing direction but not its speed. For example, a car going in a circle at constant speed has a changing velocity, but its speed remains constant.

So, to answer your question more accurately, we need to know what you're asking about:

* Are you asking when an object is at rest (not moving)? Then the speed is zero.

* Are you asking when the velocity of an object is zero? Then the speed is zero when the object is not moving and not changing direction.

Let me know if you have a specific scenario in mind, and I can give you a more precise answer!