Free and Forced Vibrations

1
Free and Forced Vibrations

• An Oscillation can be defined by the movement of
  an object ...
• Examples of Oscillators are
• ..
• The Inertia of an object can be described as
• Free and Forced Vibrations
• A Free oscillator is when an ..oscillat
  or is displaced, ( e.g. a pendulum is swung) and
  then . it to , with no..
  acting on it.
• A forced oscillator is when an ..,
  forcing .. is applied
  to the oscillating object.

2
Forced Oscillators

• Responses of forced oscillators
• If vibrations have the frequency as
  the of the object, the of
  the oscillations will ..rapidly. This Is
  called ...
• At resonance the driver has to apply the .
  force (.), to the oscillator at a
  rate, in order to .. the
  amplitude of the object, this is why
  the amplitude increases and there is no ..
• Therefore the rate of supply of .. is
  .. . to the rate of of ., as
  work is done against forces.

3
Uses of Resonance

• In order to sound heavy church bells, bell
  ringers swing them at the resonant frequency of
  the bell in its carriage.
• Resonance of strings at their .. ..
  and multiples of them give us musical sounds.
• Wind instruments are sounded by making a
  .. of air resonate by either blowing a
  whistle at one end.
• Resonance of makes radio waves and
  allows them to be received.

4
Graph!

• By plotting a graph of amplitude against
  frequency, there is a large peak, occurring at
  the . (..).

5
Limitations of Resonance

• Sometimes when there is no force to stop this
  effect, the oscillator may, if it
  continues ...
• If the frequency . to the object does not
  to the of the object,
  then the oscillation is
• The of the resonant . may be limited
  by . forces.
• thus increasing the , .. the
  sharpness and of the resonance.
• So .resonance in and machinery is
  can be .. damping .
• If an object oscillates at the
  of another, then the .. object may start to
  . .

6
Dangers of Resonance

• Panels in a bus rattling.
• Resonance in car suspensions needs to be damped.
  The shock absorbers must be working properly, or
  the car will go out of control.
• Suspension bridges can start rocking in the wind
  at their resonant frequency, and the oscillations
  can get so large that the deck can collapse into
  the sea.

7
Questions to Do !

• A rope hanging from a tree swings with a period
  of 5 s. What is its frequency?
  
  
  .
• What is the difference between natural and forced
  oscillations?
  
  ..
  ..
• At a certain engine speed, a cars wing mirror
  starts to vibrate strongly. Why does this
  happen?
  
  
  
  .
• Explain why worn shock absorbers can make a car
  fail its annual MOT (an annual check in which
  about 30 safety items are checked. It is illegal
  to drive a car without a current MOT).
  
  
  
  
  
  
  .