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    I 2
I  2  I  1   2   1  ;
2
1
I 1
(6.39)
3) а spinning figure skater reduces her moment of inertia by pulling

in her arms, causing her rotation rate to increase(fig. 6.11c) .
All saw a helicopter with two rotors as shown on fig. 6.12 Rotor 1
carry out aerodynamic lift force. If this rotor rotates with angular

velocity  , then according to the law of conservation of angular
1
momentum the helicopter should rotate in opposite direction with

angular velocity  .Rotor 2 on the tail of the helicopter counteracts
2
to rotation of the helicopter.
It is very important to note , that the law of conservation of angular
momentum is the most fundamental law of nature an acts in .physics.
However, in quantum

physics, there is another
type of angular
momentum, called spin
1
2 angular momentum,
represented by the spin
operator S. Almost all
elementary particles

have spin. Spin is often
depicted as a particle

literally spinning around
Figure 6.12
an axis, but this is a
misleading and
inaccurate picture: Spin

is an intrinsic property of a particle, fundamentally different from orbital
angular momentum. All elementary particles have a characteristic spin,
for example electrons always have "spin 1/2".

6.6 Free Axes of Rotation. Principal Axes of Inertia. Tensor of
Inertia.
6.7

The body that can rotate around the axis may be in equilibrium if
algebraic sum of moments of forces (torques) is equal to zero:

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