Create panel > Space Warps > Forces > Object Type rollout > Gravity
Create menu > Space Warps > Forces > Gravity
The Gravity space warp simulates the effect of natural gravity on particles generated by a particle system. Gravity is directional. Particles moving in the direction of the gravity arrow accelerate. Particles moving against the arrow decelerate. In the case of spherical gravity, motion is toward the icon. Gravity can also be used as an effect in dynamics simulations. See Dynamics Utility.
To create gravity:
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On the Create panel, click Space Warps. Choose Forces from the list, then on the Object Type rollout, click Gravity.
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The Gravity icon appears. For planar gravity (the default), the icon is a wireframe square with a direction arrow on one side. For spherical gravity, the icon is a wireframe sphere.
The initial direction of planar gravity is along the negative Z axis of the construction grid that is active in the viewport where you drag. You can rotate the gravity object to change the direction.
Strength—Increasing Strength increases the effect of gravity; that is, how objects move in relation to the Gravity icon's direction arrow. Strength less than 0.0 creates negative gravity, which repels particles moving in the same direction and attracts particles moving in the opposite direction. When Strength is set to 0.0, the Gravity space warp has no effect.
Decay—When Decay is set to 0.0, the Gravity space warp has the same strength throughout world space. Increasing the Decay value causes gravity strength to diminish as distance increases from the position of the gravity warp object. Default=0.0.
Planar—Gravity effect is perpendicular to the plane of the Gravity warp object throughout the scene.
Spherical—Gravity effect is spherical, centered on the Gravity warp object. This choice is effective for creating water fountain or planetary effects.
Range Indicators—When on, and when the Decay value is greater than 0.0, icons in the viewports indicate the range at which the force of gravity is half the maximum value. For the Planar option, the indicators are two planes; for use the Spherical option, the indicator is a double-hooped sphere.
Icon Size—Size of the Gravity warp object icon, in active units. You set the initial size when you drag to create the Gravity object. This value does not change the gravity effect.