1. REGENTS PHYSICS MR. BALDWIN
Resonance
REGENTS PHYSICS
MR. BALDWIN

2. AIM: Why do some tall building collapse during
an earthquake?
DO NOW: Blow over the top of an empty or even half-full bottle. What did you observe?

3. Resonance
Forced oscillations/vibrations are small unless the driving frequency is close to the natural frequency.
When the driving frequency is equal to the natural frequency the oscillations/vibrations can be large. This is called Resonance.

4. Let’s look at some examples of forced vibrations
1. Bridges: Tacoma Narrows Bridge
Crosswind in valley caused the bridge to resonate and collapse

5. Let’s look at some examples of forced vibrations
2. Vehicles
Body of bus or motorcycle vibrates when it is travelling at a particular speed.
3. Glass
A glass is broken by sound wave vibrating at resonant frequency of the glass.

6. Let’s look at what happens when we resonate a wine glass
An optical technique called interferometry reveals the oscillations of a wine glass
Let’s look at what happens when we resonate a wine glass

7. Let’s look at some examples of forced vibrations
4. Electrical circuits
Radio receiver works on principle of resonance - tune to particular station, radio waves received have same frequency as that of electrical oscillations in circuits - electrical oscillations of high amplitude and same frequency as the radio waves is induced in receiver

8. Let’s look at some examples of forced vibrations
5. Microwave ovens
- Frequency of microwaves = Natural frequency of vibration of water molecules - Water molecules in food resonate and food get heated up

9. Let’s look at some examples of forced vibrations
6. Magnetic Resonance Imaging (MRI)
- used for medical diagnoses - EM fields of varying frequencies cause nuclei of atoms to oscillate - Energy is absorbed by atoms in resonance - Pattern of energy absorption produces computer enhanced photograph

11. Probably the most famous incident in which this resonance resulted in destruction is the case of the Tacoma Narrows Bridge.

12. Unfortunately, the bridge’s natural resonant frequency was the same as the wind blowing across the Puget Sound on certain days.

13. During times when the conditions were just right, the energy from the wind was transferred to the bridge.

14. As the amplitude of the wind induced vibrations became larger and larger, the bridge ultimately gave way and collapsed.

15. Every bridge constructed vibrate at some natural, or resonant frequency.
CHECK!
How or what can engineers do to design bridges that wouldn’t resonate?
Check the rate of the bridge’s resonant frequency.