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SHM23

1.7 Forced Oscillations and Resonance                LO (j)


When a system performs oscillations without any applied force, the frequency of the oscillation is a characteristic of the system and is called the natural frequency, fo . In the example above, the fo = 0.161 Hz.

1.7.1 Spring Mass System initially at Rest

Consider a spring mass system initially at rest.

1.7.1.1 No driving force

When the driving force is at f = 0, that means the driving force is  Fdriver = (0.5)sin(2π f t ) = 0.

the result is as shown on the picture where x = 0 for all time since there is no external driving force acting on the system.

1.7.1.2 Slow driving force

However, when f = 0.1 , the Fdriver = (0.5)sin(2π (0.1) t ), it results in a motion that seems to irregular reaching a maximum amplitude of A and B (see picture).

1.7.1.3 Driving force equal Natural frequency

However, when f = 0.159 , the Fdriver = (0.5)sin(2π (0.159) t ), it results in a motion that seems to increase to a very large maximum amplitude of A and B (see picture, when t = 55.8, A and B are greater than the screen display height). Theoretically, is should be infinitely large.

1.7.1.4 Driving force greater than Natural frequency

However, when f = 0.2 , the Fdriver = (0.5)sin(2π (0.2) t ), it results in a motion that seems to also reach some smaller maximum amplitude of A and B (see picture reaching about 1.8 ).

1.7.1.5 Driving force much greater than Natural frequency

However, when f = 0.3 , the Fdriver = (0.5)sin(2π (0.3) t ), it results in a motion that seems to also reach rather maximum amplitude of A and B (see picture reaching about 0.5 ).

Lastly, when f = 0.4 , the Fdriver = (0.5)sin(2π (0.4) t ), it results in a motion that seems to also reach even smaller maximum amplitude of A and B (see picture reaching about 0.3 ).

These oscillations in which a periodic force is imposed are called forced resonance. Systems will then some what influenced to vibrate at the frequency of the periodic force, f, but only when f = fo = 0.161 Hz , in this example, the maximum amplitude is the greatest happens in this case when damping is zero as has similar maximum amplitude effects for different damping levels occurring near f = fo

1.7.2 Examples include the

1)    Vibrations of a pendulum clock.
2)    Vibrations of a tuning fork when exposed to the periodic force of a sound wave.
3)    Vibration of a bridge under the influence of marching soldiers or periodic wind.

1.7.3 Model:

  1. Run Sim
  2. http://iwant2study.org/ospsg/index.php/87
 

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end faq

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Apps

Cover arthttps://play.google.com/store/apps/details?id=com.ionicframework.shm23app540489&hl=en

end faq

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