We will probably discover that the parts fail under the dynamic forces. Then we will
have to go back to our original assumptions about the shapes, sizes, and materials of
these parts, redesign them, and repeat the force, stress, and deflection analyses. Design
is, unavoidably, an iterative process.
The topic of stress and deflection analysis is beyond the scope of this text and will
not be further discussed here. It is mentioned only to put our discussion of dynamic force
analysis into context. We are analyzing these dynamic forces primarily to provide the
information needed to do the stress and deflection analyses on our parts! It is also worth
noting that, unlike a static force situation in which a failed design might be fixed by adding
more mass to the part to strengthen it, to do so in a dynamic force situation can have
a deleterious effect. More mass with the same acceleration will generate even higher
forces and thus higher stresses! The machine designer often needs to remove mass (in
the right places) from parts in order to reduce the stresses and deflections due to F = ma.
Thus the designer needs to have a good understanding of both material properties and
stress and deflection analysis to properly shape and size parts for minimum mass while
maximizing the strength and stiffness needed to withstand the dynamic forces.