Problem while creating balancing board using Unity physics

Question

I am trying to create a balancing board using Unity Physics. Below are the steps that I followed to achieve my result and I got unexpected Unity Physics.

1. Balancing-bar and base are connected through a sphere and sphere is having a fixed joint with balancing-bar and I freeze y,z rotation and all the positions of the sphere and base's all position and rotation are fixed.

2. Balancing-bar is having a rigid body with Mass, Drag and Angular Drag 5,1,5 respectively and rotation on y and z are Freeze. Balancing bar is connected to the sphere using Hinge Joint. below I attached an Image for understanding.

I created a video for understanding.

Video Link

Problems:
1. When I throw an object a bit above from my balancing-board then it that side of bar automatically pulls down.

(Both objects is having the same mass=1 and both are 4 Meter far from the pivot point and on "Let's check" button I just turn off board's isKinematic)

2. When I do the same before Unlocking the isKinematic it works perfectly well.

I am just blank here what to do next or where I did the mistake, any help is appreciated.

Answer 1

To create a weighing balance that will return to the original position, the horizontal beam must be positioned below the axis of rotation. Read more here.

In your case, the center of mass for the boxes plus the beam is above the rotational axis. The net force will not help move the beam toward the original position, as a weighing balance does.

I see at least two ways of fixing this. One way is to change your seesaw construction into a weighing balance, with the boxes hanging down below the beam. Another way would be to shift the center of mass for the boxes below the beam. In a unity script, do something like this:

rigidbody.centerOfMass = Vector3 (0, -2, 0);

You might need to shift the CM more or less depending on your setup. I've added a GIF showing a that the shifted center of mass approach works (using p2.js physics engine). The small circles at the bottom are the center of mass positions for the boxes.