Vibration is essentially a to-and-fro motion. Thus there is a force (excitation/disturbance) that initiates the motion. We will learn about different types of excitation forces in the next lecture. Under the influence of the external disturbance excitation, the system masses move i.e., they accelerate and decelerate setting up inertia forces. If external excitation were the only type of force on the system, the system would exhibit a rigid body motion. However since most systems are elastic, the movement of the masses invariably causes stretching or compression of springy elements setting up elastic restoring forces.
For example when an automobile passes on a road, the road roughness is the external excitation. The mass of the vehicle moves up-down (pitch and bounce), left-right (roll) setting up inertia forces. The suspension spring gets stretched and compressed as the vehicle mass moves up and down. When a spring is stretched or compressed from its free length position, it exerts a restoring force on the mass trying to bring it back to its free length position. In the process of course the mass would have gained momentum and continues to travel farther than the static equilibrium, free length position. Once again the spring tries to pull the mass back to its free length position and the cycle repeats.
The cycle of to-and-fro motions would however not repeat forever due to the dissipation present in most systems. For example, an automobile suspension always has a shock absorber i.e., a damper that dissipates the energy of vibration into friction against a moving fluid. |
