Mass Pivot and Rigid Body

Each 3D entity has its own pivot, which is the base for prop-puppeteering, transforming, rotating and scaling. When generating physics animations, your prop also has one more pivot, which is called the Mass Pivot. These two pivots can be located at different positions of your prop, and the Mass Pivot has a strong influence on the physics rotation of any rigid body object.

The Rotation of a Rigid Body

When a rigid body rotates, it's bounding mesh determines the angle of collision, bouncing, and friction. It is also affected by the Mass Pivot. It is forced to fall along gravity's direction until the bounding box is frozen via a state change to a static object.

Friction, collision and bouncing are generated by the bounding mesh of the object.	Gravity forces the mass pivot to drag the object down towards the ground.

Mass Pivot and Rolling

The position of the mass pivot affects the rolling behavior of all objects in any shape, especially cylinder and capsules. If the mass pivot of an object is in a strange place, the object may never appear to tumble.

1. Apply a cylinder. Rotate it and set it as a Dynamic rigid body.
   
   When you hit the play button to simulate the physics animation, you can expect to see the cylinder fall to the floor as shown below:
   
2. Play to simulate. You will see that the cylinder stands up instead of lying down on the floor.

   The real simulation result.

   • This is because the mass pivot is at the center of the circle on the dark side of the cylinder; which pulls the cylinder and makes it topple.

     The mass pivot is pulled by gravity.	The real simulation.

3. To fix this issue, you may simply set the mass pivot to the center of the cylinder.

   Move the mass pivot to the center of the object.	The real simulation.

Relocating Mass Pivot - Creating a Balancing Toy

Although the rotation looks awkward when the mass pivot is eccentric, you may utilize it to create a balancing toy.

Because the initial mass pivot is located in the bottom or center of the object, it will cause the toy in the following example to fall directly.

The mass pivot is at the center of the object. The gravity pulls the object via its mass pivot.	The object falls.

1. Select the object.
2. In the Modify >> Physics tab, activate the Activate Physics and choose the Rigid Body radio button to make the object a physics one.
3. Press down the Edit Offset button in the Center of Mass section.
   
4. Offset the pivot lower than the bottom of the prop by entering values to the X, Y and Z fields.
   
   
   Note:

   Optionally turn on the gizmo mode (Ctrl + Q) if you want to use the gizmo to adjust the position of the mass pivot.
   

5. Play back to start the simulation.

   Deflection formed by gravity's direction and the object's blue axis.	The pivot rectifies the object's orientation and deflection.