# Kinematics and Kinetics

Kinematics: The study of motion without regard to forces.
Kinetics: The study of forces on systems in motion.

These two concepts are really not physically separable. One cannot design most dynamic mechanical systems without taking both topics into thorough consideration.

It is also valid in engineering design practice to first consider the desired kinematic motions and their consequences, and then subsequently investigate the kinetic forces associated with those motions.

From Newton’s second law, F = ma, one typically needs to know the accelerations (a) in order to compute the dynamic forces (F) due to the motion of the system’s mass (m). There are also many situations in which the applied forces are known and the resultant accelerations are to be found.

One principal aim of kinematics is to create (design) the desired motions of the subject mechanical parts and then mathematically compute the positions, velocities, and accelerations which those motions will create on the parts. Since, for most earthbound mechanical systems, the mass remains essentially constant with time, defining the accelerations as a function of time then also defines the dynamic forces as a function of time. Stresses, in turn, will be a function of both applied and inertial (ma) forces.

Since engineering design is charged with creating systems which will not fail during their expected service life, the goal is to keep stresses within acceptable limits for the materials chosen and the environmental conditions encountered. This obviously requires that all system forces be defined and kept within desired limits. In machinery which moves (the only interesting kind), the largest forces encountered are often those due to the dynamics of the machine itself. These dynamic forces are proportional to acceleration, which brings us back to kinematics, the foundation of mechanical design.

Very basic and early decisions in the design process involving kinematic principles can be crucial to the success of any mechanical design. A design which has poor kinematics will prove troublesome and perform badly.

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