Question

What are forced vibrations? Give one example to illustrate your answer.

Answer 1

Hint: Like the name suggests, forced vibrations are due to the external forces. These vibrations will get transferred from the source to the surroundings. That’s why we are getting shivering when we touch a washing machine during its working.

Complete step-by-step answer:
When a body undergoes vibrations under the influence of an external force, then the body is said to be in forced vibrations. Due to the external force, the body does not vibrate with its own natural frequency, but it vibrates with the frequency generated by the applied external force. Periodic forces, impulsive type of forces and random forces are the reasons behind the external excitation.
We can take washing machines as an example for forced vibrations. Electric motors or rotating machines still have some imbalance even after the rectification. For our case, if we are depositing clothes in the washing machine. During rotation. probably there will be an unbalance in rotating mass. This unbalance will produce a force known as centrifugal force, which acts like an external force and causes forced vibrations on the machines. These vibrations can be felt from the floor where the washing machine attached. Vibration of vehicles during the running on uneven roads, vibration of air compressors and musical instruments etc. are some of the examples for forced vibrations.

Additional information:
Vibrations developed in a machine due to external forces will get transmitted into the base where the machine is attached. Due to the undesired effects of forced vibrations, we need to avoid this. These vibrations can be isolated by using springs and dampers. This is known as vibration isolation. Transmissibility is the way used to measure the amount of transferred vibrations to the foundation.
Free vibrations are also possible during daily life. These vibrations occur in the absence of external forces like friction after the release of the body. Simple pendulum is an example for this.

Note: As we know, all vibrations are not forced vibrations. The swing and simple pendulum are working with free vibrations. In these cases, we won’t apply any external forces after the release of the body. But if we are applying any forces during the swing, that will not be the free vibrations.