# Rotation by inertia

Indeed, untwisted carousel, - and turn yourself by inertia. But **rotation by inertia** Is it? And what's the difference between inertia of rectilinear and rotational motion?

We will carry out the following experiment. We will try to rotate a rod around the vertical axis with masses (weights) loaded on it, for example, with metal balls. While these balls are near the center, spinning the rod is easy, inertia is small. But if we push the masses to the edges of the rod, then it will become much harder to unleash such a rod, although its mass remained unchanged. Consequently, the inertia of the body during rotation depends not only on the mass, but also (even more so) on the distribution of these masses relative to the axis of rotation. The measure of the inertia of the body during rotation is the so-called moment of inertia. Thus, the difference in the measure of inertia of rectilinear motion and rotation is that in the first case it is measured by mass, and in the second case by the moment of inertia.

The moment of inertia of a body with respect to a given axis is a quantity equal to the sum of the products of the masses of all particles of the body and the squares of their distances from this axis.

As we know, the law of inertia establishes the equivalence of relative rest and uniform rectilinear motion - motion by inertia. For it is impossible to establish by any mechanical experiment whether the given body is at rest or moves uniformly and rectilinearly. In rotational motion, this is not so. For example, it is not at all indifferent whether the top is at rest or rotates uniformly, with a constant angular velocity. The angular velocity of a solid is a quantity that characterizes its physical state. If even the entire Universe disappears, leaving only our rotating body, then in this case we will also know its angular velocity.

Therefore, the second difference is that rectilinear motion and rest are equivalent, and rotation, even at a constant angular velocity, can be clearly separated not only from rest, but also from rotation with another angular velocity.

Here, perhaps, and all the main differences. The rest is so identical that one can take the liberty to formulate the "law" of inertia of the rotational motion of an absolutely rigid body: "An absolutely rigid body isolated from external moments will maintain a state of rest or uniform rotation around a fixed point or axis until the moments attached to the body external forces will not make him change this state".

On the phenomenon of inertia of rotational motion, numerous instruments and machines are based, in particular, inertial engines - accumulators, which retain kinetic energy during inertial rotation of the flywheel, and gyroscopic devices.