Forced oscillations Resonance_Presentation

The Millennium Footbridge in London, opened in June 2000. After three days the bridge was closed. Why?

https://www.youtube.com/watch?v=eAXVa__XWZ8

Barton’s pendulums.

https://www.youtube.com/watch?v=SS1kmiNFbgQ

Forced oscillations. Resonance.Uses/advantages and disadvantages of resonance

11 grade 2018-2019

Describe graphically how the amplitude of a forced oscillation changes with frequency near to the natural frequency of the system, and understand qualitatively the factors that determine the frequency response and sharpness of the resonance;
Show an appreciation that there are some circumstances in which resonance is useful and other circumstances in which resonance should be avoided

Undamped
Natural frequency, f0
NO External Forces
E.g playground swing

External forces act as an oscillator
Forced/Driven oscillator
Resonance: driving f=f0
E.g Loudspeaker vibrates in response to an oscillating driving signal (driver)
Barton’s Pendulums

Ideal oscillation

These are vibrations that are driven by an external force.

Resonance is the increase in amplitude of oscillation of a system, when the frequency of the driving force equals (or very close) the natural frequency (f0 ) of the system.

The force does additional work affecting the total mechanical energy. Hence, the amplitude increases.

Determine what are the factors affecting resonance.

https://phet.colorado.edu/en/simulation/resonance

Activity Sheet_Resonance

1. Which system will have the lower resonance frequency?

2. Which system will have the lower resonance frequency?

3. Which system will have the lower resonant frequency?

4. Two resonators are driven at the same driving frequency and amplitude. One resonator has a resonant frequency 2 Hz below the driving frequency. The other has a resonant frequency 2 Hz above the driving frequency.

Suppose the system is acted upon by a harmonically varying external force, called the “driving force”. The frequency of the driving force is f. When f = f0 , the amplitude of the oscillations grows dramatically.

As the degree of damping is increased, the amplitude of the resonant vibrations
decreases. The resonance peak becomes broader.

There is an effect on the frequency at which resonance
occurs, which becomes lower

https://www.youtube.com/watch?v=uP3J_Ad2Zhw

Musical instruments rely on resonance

Microwave cooking
The microwaves used have a frequency that matches a natural frequency of vibration of water molecules

Radio waves having a range of frequencies are used, and particular frequencies are
absorbed by particular atomic nuclei.

The tuner can be adjusted to resonate at the frequency of the transmitting station you are interested in, and the circuit produces a large-amplitude signal for this frequency only.

https://www.youtube.com/watch?time_continue=6&v=iak1VuJyyoM

Individual work. Solve problems.
Worksheets CIE Paper 2
9702/41/M/J/15
9702/41/O/N/10
20-25 min

Describe graphically how the amplitude of a forced oscillation changes with frequency near to the natural frequency of the system, and understand qualitatively the factors that determine the frequency response and sharpness of the resonance;
Show an appreciation that there are some circumstances in which resonance is useful and other circumstances in which resonance should be avoided