# changeMass(by:duration:) Creates an action that changes the mass of a node’s physics body by a relative value. ``` class func changeMass(by v: Float, duration: TimeInterval) -> SKAction ``` ## Parameters `v` The amount to add to the physics body’s mass. `duration` The duration of the animation. ## Return Value A new action object. ## Discussion When the action executes, the physics body’s [`mass`](/documentation/SpriteKit/SKPhysicsBody/mass) property animates from its current value to its new value. This action is reversible; the reverse is created as if the following code is executed: ```swift let action = SKAction.changeMass(by: -v, duration: sec) ``` --- 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)