## Wednesday, March 10, 2010

### [Physics Form 5] Power Of Resonance

Resonance is what happens when the vibrations produced by one object come into alignment with those another. An apparatus set-up is shown as below.

When the radio is switched on, the diaphragm is observed to vibrate. A few moments later, the polystyrene ball starts to oscillate. It oscillates as long as the radio switched on. When the radio is switched off, the amplitude of oscillating polystyrene ball decreases and eventually becomes zero.

Q & A
1. Q: How is the polystyrene ball able to oscillate when no apparent external force is present to displace and release it?
A: The vibration of the loudspeaker produces energy, which is transferred to the air molecules in front of it. The energy is then transferred from one air molecule to another until it reaches the polystyrene ball. When the polystyrene ball receives the energy, it is displaced from its equilibrium position, thus setting it into oscillation. This shows that the oscillating system transfers its energy to its surrounding medium.

2. Q: Why is a polystyrene ball used? What if the polystyrene ball is substituted with a lead bob?
A: Polystyrene is lighter or less massive than most materials. If a lead bob is used, no oscillation will be observed. A lead bob, with a relatively larger mass, generates larger inertia. Hence, the small amount of energy transferred by the loudspeaker is not able to overcome the inertia and displace the bob to set it into oscillation.

Did You Know?

• The ocean-like sound we hear when we place a seashell to our ears is due to an effect of resonance.

• Resonance can also damage properties. The collapse of the Tacoma Narrows Bridge in Washington in 1940 was a result of resonance. The frequency of the wind was equal to the natural frequency of the bridge, thus setting the structure into vibration, and causing it to collapse.