### Two Masses Connected by a Rod

As an introduction to the decomposition of rigid-body motion into

*translational*and

*rotational*components, consider the simple system shown in Fig.B.5. The excitation force density

^{B.15}can be applied anywhere between and along the connecting rod. We will deliver a vertical impulse of momentum to the mass on the right, and show, among other observations, that the total kinetic energy is split equally into (1) the

*rotational*kinetic energy about the center of mass, and (2) the

*translational*kinetic energy of the total mass, treated as being located at the center of mass. This is accomplished by defining a

*new frame of reference*(

*i.e.*, a moving coordinate system) that has its origin at the center of mass. First, note that the driving-point impedance (§7.1) ``seen'' by the driving force varies as a function of . At , The excitation sees a ``point mass'' , and no rotation is excited by the force (by symmetry). At , on the other hand, the excitation only sees mass at time 0, because the vertical motion of either point-mass initially only rotates the other point-mass via the massless connecting rod. Thus, an observation we can make right away is that the

*driving point impedance seen by depends on the striking point and, away from , it depends on time as well*. To avoid dealing with a time-varying driving-point impedance, we will use an impulsive force input at time . Since momentum is the time-integral of force ( ), our excitation will be a

*unit momentum*transferred to the two-mass system at time 0.

#### Striking the Rod in the Middle

First, consider . That is, we apply an upward unit-force impulse at time 0 in the middle of the rod. The total momentum delivered in the neighborhood of and is obtained by integrating the applied force density with respect to time and position:*velocity of the center of mass*, again obvious by symmetry. Continuing to refer to Fig.B.5, we have

#### Striking One of the Masses

Now let . That is, we apply an impulse of vertical momentum to the mass on the right at time 0. In this case, the unit of vertical momentum is transferred entirely to the mass on the right, so that*same*as it was when we hit the midpoint of the rod. This is an important general equivalence: The sum of all external force vectors acting on a rigid body can be applied as a single resultant force vector to the total mass concentrated at the center of mass to find the linear (translational) motion produced. (Recall from §B.4.1 that such a sum is the same as the sum of all radially acting external force components, since the tangential components contribute only to rotation and not to translation.) All of the kinetic energy is in the mass on the right just after time zero:

However, after time zero, things get more complicated, because the mass on the left gets dragged into a rotation about the center of mass. To simplify ongoing analysis, we can define a

*body-fixed frame of reference*

^{B.16}having its origin at the center of mass. Let denote a velocity in this frame. Since the velocity of the center of mass is , we can convert any velocity in the body-fixed frame to a velocity in the original frame by adding to it,

*viz.*,

*rotational*about the origin. Recall (Eq.(B.9)) that the moment of inertia for this system, with respect to the center of mass at , is

*rotational kinetic energy*(§B.4.3) is found to be

*half*of the kinetic energy we computed in the original ``space-fixed'' frame (Eq.(B.13) above). The other half is in the

*translational kinetic energy*not seen in the body-fixed frame. As we saw in §B.4.2 above, we can easily calculate the translational kinetic energy as that of the total mass traveling at the center-of-mass velocity :

*constant*over time.

^{B.17}In the original space-fixed frame, on the other hand, there is a complex transfer of momentum back and forth between the masses after time zero. Similarly, the translational kinetic energy of the total mass, treated as being concentrated at its center-of-mass, and the rotational kinetic energy in the body-fixed frame, are both constant after time zero, while in the space-fixed frame, kinetic energy transfers back and forth between the two masses. At all times, however, the total kinetic energy is the same in both formulations.

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Angular Velocity Vector

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Radius of Gyration