# König's theorem (kinetics)

König's theorem (kinetics) For other uses, see König's theorem (disambiguation).

In kinetics, König's theorem or König's decomposition is a mathematical relation derived by Johann Samuel König that assists with the calculations of angular momentum and kinetic energy of bodies and systems of particles.

Contents 1 For a system of particles 1.1 First part of König's theorem 1.1.1 Proof 1.2 Second part of König's theorem 1.2.1 Proof 2 For a rigid body 3 References 4 Works Cited For a system of particles The theorem is divided in two parts.

First part of König's theorem The first part expresses the angular momentum of a system as the sum of the angular momentum of the centre of mass and the angular momentum applied to the particles relative to the center of mass. [1] {displaystyle displaystyle {vec {L}}={vec {r}}_{CoM}times sum limits _{i}m_{i}{vec {v}}_{CoM}+{vec {L}}'={vec {L}}_{CoM}+{vec {L}}'} Proof Considering an inertial reference frame with origin O, the angular momentum of the system can be defined as: {displaystyle {vec {L}}=sum limits _{i}({vec {r}}_{i}times m_{i}{vec {v}}_{i})} The position of a single particle can be expressed as: {displaystyle {vec {r}}_{i}={vec {r}}_{CoM}+{vec {r}}'_{i}} And so we can define the velocity of a single particle: {displaystyle {vec {v}}_{i}={vec {v}}_{CoM}+{vec {v}}'_{i}} The first equation becomes: {displaystyle {vec {L}}=sum limits _{i}({vec {r}}_{CoM}+{vec {r}}'_{i})times m_{i}({vec {v}}_{CoM}+{vec {v}}'_{i})} {displaystyle {vec {L}}=sum limits _{i}{vec {r}}'_{i}times m_{i}{vec {v}}'_{i}+left(sum limits _{i}m_{i}{vec {r}}'_{i}right)times {vec {v}}_{CoM}+{vec {r}}_{CoM}times sum limits _{i}m_{i}{vec {v}}'_{i}+sum limits _{i}{vec {r}}_{CoM}times m_{i}{vec {v}}_{CoM}} But the following terms are equal to zero: {displaystyle sum limits _{i}m_{i}{vec {r}}'_{i}=0} {displaystyle sum limits _{i}m_{i}{vec {v}}'_{i}=0} So we prove that: {displaystyle {vec {L}}=sum limits _{i}{vec {r}}'_{i}times m_{i}{vec {v}}'_{i}+M{vec {r}}_{CoM}times {vec {v}}_{CoM}} where M is the total mass of the system.

Second part of König's theorem The second part expresses the kinetic energy of a system of particles in terms of the velocities of the individual particles and the centre of mass.

Specifically, it states that the kinetic energy of a system of particles is the sum of the kinetic energy associated to the movement of the center of mass and the kinetic energy associated to the movement of the particles relative to the center of mass.[2] {displaystyle K=K'+K_{text{CoM}}} Proof The total kinetic energy of the system is: {displaystyle K=sum _{i}{frac {1}{2}}m_{i}v_{i}^{2}} Like we did in the first part, we substitute the velocity: {displaystyle K=sum _{i}{frac {1}{2}}m_{i}|{bar {v}}'_{i}+{bar {v}}_{text{CoM}}|^{2}} {displaystyle K=sum _{i}{frac {1}{2}}m_{i}({bar {v}}'_{i}+{bar {v}}_{text{CoM}})cdot ({bar {v}}'_{i}+{bar {v}}_{text{CoM}})=sum _{i}{frac {1}{2}}m_{i}{v'_{i}}^{2}+{bar {v}}_{text{CoM}}cdot sum _{i}m_{i}{bar {v}}'_{i}+sum _{i}{frac {1}{2}}m_{i}v_{text{CoM}}^{2}} We know that {displaystyle {bar {v}}_{CoM}cdot sum _{i}m_{i}{bar {v}}'_{i}=0,} so if we define: {displaystyle K'=sum _{i}{frac {1}{2}}m_{i}{v'_{i}}^{2}} {displaystyle K_{text{CoM}}=sum _{i}{frac {1}{2}}m_{i}v_{text{CoM}}^{2}={frac {1}{2}}Mv_{text{CoM}}^{2}} we're left with: {displaystyle K=K'+K_{text{CoM}}} For a rigid body The theorem can also be applied to rigid bodies, stating that the kinetic energy K of a rigid body, as viewed by an observer fixed in some inertial reference frame N, can be written as: {displaystyle ^{N}K={frac {1}{2}}mcdot {^{N}mathbf {bar {v}} }cdot {^{N}mathbf {bar {v}} }+{frac {1}{2}}{^{N}!mathbf {bar {H}} }cdot ^{N}{!!mathbf {omega } }^{R}} where {displaystyle {m}} is the mass of the rigid body; {displaystyle {^{N}mathbf {bar {v}} }} is the velocity of the center of mass of the rigid body, as viewed by an observer fixed in an inertial frame N; {displaystyle {^{N}!mathbf {bar {H}} }} is the angular momentum of the rigid body about the center of mass, also taken in the inertial frame N; and {displaystyle ^{N}{!!mathbf {omega } }^{R}} is the angular velocity of the rigid body R relative to the inertial frame N.[3] References Hanno Essén: Average Angular Velocity (1992), Department of Mechanics, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Samuel König (Sam. Koenigio): De universali principio æquilibrii & motus, in vi viva reperto, deque nexu inter vim vivam & actionem, utriusque minimo, dissertatio, Nova acta eruditorum (1751) 125-135, 162-176 (Archived). Paul A. Tipler and Gene Mosca (2003), Physics for Scientists and Engineers (Paper): Volume 1A: Mechanics (Physics for Scientists and Engineers), W. H. Freeman Ed., ISBN 0-7167-0900-7 Works Cited ^ Essén, Hanno (1993). "Average Angular Velocity". European Journal of Physics. 14 (5): 201–205. arXiv:physics/0401146. Bibcode:1993EJPh...14..201E. doi:10.1088/0143-0807/14/5/002. ^ Essén, Hanno (1993). "Average Angular Velocity". European Journal of Physics. 14 (5): 201–205. arXiv:physics/0401146. Bibcode:1993EJPh...14..201E. doi:10.1088/0143-0807/14/5/002. ^ Rao, Anil V. Dynamics of Particles and Rigid Bodies: A Systematic Approach. Cambridge University Press. p. 421.