Kinematics

In a theory of mechanics, kinematics is the quantitative description of the configuration and trajectory of the system in question, without reference to the force or forces involved.

Examples

• In Newtonian Mechanics, the kinematic specification of a problem is a set of times, dimensions, positions, velocities and accelerations for each of a collection of bodies, which describes quantitatively the trajectory of the system.
  • The kinematics of a structural truss, for example, is a specification of its configuration, dimensions, geometrical aspects of support, geometrical aspects of loading and, in some cases, deflections. On this kinematic basis, the mechanical analysis can yield the force in each link according to the magnitudes of the loads.
  • The kinematics of a railcar can be a specification of the distance of the car, from a fixed point on the track, as a function of time. In another problem referring to the same railcar, the kinematics can be the distance of the car from a point on the track at one specified time, along with the acceleration of the car as a function of time. In each of these problems, the kinematic specification allows calculation of the force acting on the car as a function of time.

• In thermal mechanics, the kinematic specification is a description of the configuration of the system along with a set of volumes, flow rates and temperatures and characteritics of the thermal medium. This specifies a state or trajectory of the system while additional information of pressure or power input is required to fully determine the behaviour of the system.
  • For example, the kinematics of a gas in a network of connected chambers is the specification of elevation, temperature and volume of each chamber. With this information and the pressure in one of the chambers, the pressure in each other chamber can be calculated.
  • For a problem of a heat pump, a kinematic specification can comprise a diagram illustrating the circuit of compressor, condenser, expansion valve and evaporator along with the thermal characteristics of the refrigerant. Given the power input to the compressor, the rate of heat transfer at the evaporator can be calculated.