Biomedical Engineering Reference
In-Depth Information
Chapter 4
Rotations
Rotational motions have their origin in net external torque or are modified by
it. The difficulty in changing the state of rotation of a body depends not only on
its mass but also on how this mass is distributed about the axis of rotation. For two
bodies of the same mass, the body with the mass distributed closer to the axis of
rotation will present less resistance to change in its rotational motion.
4.1 Objectives
• To investigate the physical quantities of moment of inertia and angular
momentum
• To analyze the physical quantities that are conserved in rotations
• To relate the change in the angular velocity to modifications in the mass
distribution of a rotational system
• To investigate these quantities in the human body
4.2 Moment of Inertia
In activities of athletes of different modalities, of circus artists, of ballet dancers, or
of skaters, rotational motions are very common. Besides, all wheels of cars, of
bicycles, and of many electric devices, such as mixers and ventilators, also work
based on rotational motions. In this chapter, let us apply the concepts of physics to
explain some of the characteristics related to rotational motions.
In translational motions, the state of motion of a body only changes if a nonzero
resultant force
F
1
acts on it. In other words, we can say that if a body is at rest,
1
In this chapter we are using only the magnitude of vectorial quantities such as force, velocity,
acceleration, torque, and angular momentum. Therefore, they appear written in italics, but not in
boldface.
