The inertial forces are the fictitious forces that act on bodies when they're in motion. This property allows them to store kinetic energy.
We have been using this property of matter almost since the invention of the wheel. A famous example is a potter's wheel, which stores the kinetic energy so the wheel keeps rotating without the need for a driving machine.
We continue to use this concept in modern machines such as automobiles. The energy created by an internal combustion engine (ICE) is stored in a flywheel so we don't have to stop the engine while changing gears. A clutch helps us disengage the transmission while we're changing gears. In the meantime the flywheel stores the energy generated by the ICE.
Recently, we have even used this concept in toys such as the fidget spinner. The weight of the metal lobes stores the kinetic energy and that keeps it spinning on the bearing, which minimizes energy losses due to friction.
In this lesson we'll spend some time learning about inertial effects and how they are calculated in a time-domain analysis.
Here are the accompanying handout slides.