# velocity The initial velocity vector of the particle in world space units per second. ``` var velocity: SIMD3 { get set } ``` ## Discussion This determines the particle’s initial direction and speed of movement. The magnitude of the vector represents the speed, while the direction represents the movement direction. A zero velocity means the particle starts stationary. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)