The vibration in which a body oscillates under the effect of an external periodic force, whose frequency is different from the natural frequency of oscillating body are called forced oscillations. In forced vibrations, the oscillating body vibrates with frequency of external force and amplitude oscillations is generally small.
Examples of Forced Oscillations in Everyday Life
1. Guitar Strings
When a guitarist plucks a string, it vibrates. But the sound you actually hear is made stronger because the vibration forces the hollow body of the guitar (or an electric pickup) to oscillate too. The string’s motion drives the rest of the instrument, producing a clear, amplified tone.
2. Microphone Diaphragm
When you talk into a microphone, sound waves from your voice push against the thin diaphragm inside. Those pushes force the diaphragm to vibrate in step with your speech. Without your voice acting as the driver, the diaphragm would stay still.
3. Car Suspension
When you are driving on a rough road, every bump forces the car’s suspension to move up and down. The springs and shocks don’t bounce on their own—the road acts as the external pushing force, constantly feeding energy into the system. That’s why the ride can feel bouncy on uneven terrain.
4. Washing Machine Drum
During the spin cycle, the motor powers the drum to rotate back and forth at a steady rhythm. The clothes inside don’t set the drum in motion; it’s being driven by the machine’s motor. This is a classic case of an outside force controlling the oscillations.
5. Swing Being Pushed
Have you ever pushed a child on a swing and noticed it slows down if you stop? The swing won’t keep moving forever on its own because it loses energy little by little. But each time you give it a push, you’re adding energy from outside, forcing it to keep moving back and forth. That repeated help is what makes the swing another good example of forced oscillation.
6. Earthquake and Buildings
When an earthquake shakes the ground, buildings above it are forced into vibrations. The swaying isn’t the building’s natural choice but the ground underneath applying energy repeatedly. The longer the shaking continues, the more pronounced the building’s oscillations become.
7. Flag in the Wind
On a windy day, watch a flag closely. Each gust of wind doesn’t just move it once—the constant pressure and flow of air keep forcing the cloth to flap. Without the wind’s driving force, the flag would hang limp and motionless.
8. Bridge Vibrations from Traffic
Every truck or car that rolls across a bridge adds a little push to the structure. These repeated forces from heavy vehicles cause the bridge to oscillate slightly up and down. The motion is being driven continuously by outside traffic.
9. Human Vocal Cords
Your own voice is powered by forced oscillations. Air from your lungs pushes through your vocal cords, making them vibrate. The cords wouldn’t hum on their own. The steady airflow drives them into motion, allowing you to talk or sing.
10. Car Engine Vibrations
Have you noticed how the steering wheel or dashboard sometimes vibrates when the car is idling? The vibration is caused by the engine pistons which are firing rhythmically, forcing the parts of the car to vibrate along. The repeated pulses from the engine keep those vibrations going.