Knowing the direction of a force is important because it helps someone know the motion of the object. if you use a free body diagram, then it becomes easy to see all the forces being applied to an object. if there is more force going one way, the object is accelerating in that direction. if all the forces cancel each other out, then the object is at a constant speed or is at rest.
The angular momentum of a rotation object is the product of its moment of inertia and its angular velocity:
L = Iω
L is the angular momentum, I is the moment of inertia, and ω is the angular velocity.
Apply the conservation of angular momentum. The total angular momentum before disks A and B are joined is:
L_{before} = (3.3)(6.6) + B(-9.3)
L_{before} = -9.3B+21.78
where B is the moment of inertia of disk B.
The total angular momentum after the disks are joined is:
L_{after} = (3.3+B)(-2.1)
L_{after} = -2.1B-6.93
L_{before} = L_{after}
-9.3B + 21.78 = -2.1B - 6.93
B = 4.0kg·m²
The moment of inertia of disk B is 4.0kg·m²
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