Post on 13-Apr-2018
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6.1
WAVEDAMPING AND RESONANCE
IN AN OSCILLATING SYSTEMS
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LEARNING OUTCOMES
By the end of this lesson the student should
be able to
• Describe damping in an oscillating system
Describe resonance in a oscillating system• Describe resonance in a oscillating system
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• What can you say about the amplitude of the
swing?
• What can you do to maintain or increase the
amplitude?
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• At the start, the pendulum oscillateswith maximum amplitude.
• The amplitude of the oscillationdecreases with time and finally stops.
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What causes the pendulum to stop?
• Air friction causes the amplitude of thependulum to decrease.
• When the pendulum oscillates, it has energy.
• This energy is used to overcome air friction.
• s me passes, more energy o e pen u umis being used to overcome air friction.
• This causes the energy of the pendulum todecreases.
•
As a result, the amplitude becomes smaller.• The pendulum is said to experience damping.
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Damping
• Damping is the decrease in the amplitude of
an oscillating system when its energy is lost
as heat energy.
gradually decrease and become zero when
the oscillation stops.
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Activity 2:
Damping effect in a
vibrating system• Set up the apparatus as
shown below.
• Fix a saw bladehorizontally on the leg of
a G-clamp.
• At the other end of theblade, fix a piece of plasticine (50 g)
• Move the plasticine tothe position X andrelease it.
• Observe the position Y
reached by the plasticine.
•
Observe subsequent Ypositions of the plasticine.
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Observation:
What changes do you
observe in the positionY over several
oscillations?
Position Y decreases
The position Y becomes _________ to the
equilibrium position each time the plasticine
oscillates.
closer
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• Why does the spring oscillate closer and closer to theequilibrium position?
• Its amplitude decreases.• What happens to the energy possessed by the
plasticine and the blade?
• Energy possess decreases and lost to heat.
•
What ha ens to the oscillation of the blade after along time?
• It will stop.
• What is the name given to this phenomenon?
• Damping
• How do you overcome the damping effect of theoscillation
• Give external force or push the plasticine ball.
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• A mother puts her
baby to sleep in a
sarong cradle. Sheneeds to
continuously move
the sarong cradle up
and down.
What happen to the sarong cradle
when the mother stops moving the
cradle up and down?
• Amplitude decreases
Suggest a reason for the
movement of the sarong cradle
discussed in (a).
• Damping, ost energy to eat
What must you do if you want to
help the mother to maintain the
movement of the cradle?
• Move the cradle up and down/ give external force
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Forced oscillation & Damping
• The swing will stop
swinging due to
damping.
• The boy is given a
periodic push at the
correct timing will
continue to swingwithout stopping.
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• A system oscillates at
its natural frequency
when no external
force is applied to it.
•
When a eriodic force
•
If a periodic force isapplied to the swing
at its natural
frequency, the
am litude of the
Forced oscillation & Damping
is applied to an
oscillating system, the
response depends on
the frequency of the
periodic force.
swing increases as the
energy of the swing
increases.
•
The swing is said tobe in resonance and it
oscillates with a
maximum amplitude.
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Resonance in an oscillating system
• To enable an oscillating system to go oncontinuously, an external force must beapplied to the system.
• e ex erna orce supp es energy o esystem. Such a motion is called a forcedoscillation
• Natural frequency is the frequency of asystem which oscillates freely without theaction of an external force.
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Resonance in an oscillating systemResonance occurs when a system is made to
oscillate at a frequency equivalent to its
The resonating system oscillates at its
maximum amplitude.
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Barton’s pendulum
• The frequency of a simple pendulum depends
on the length of the pendulum.
• In Barton’s pendulum experiment, there are
many pendulums tied to the rope. Two of
• When pendulum B oscillates, all the other
pendulums are forced to
oscillate.
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Barton’s pendulum
• But pendulum D oscillates with the largest
amplitude, ie, pendulum D resonates
• Pendulum B and pendulum D are of the same
length.
• Therefore, pendulum B causes pendulum D
to oscillate at its natural frequency.
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Effects of resonance
•
Breaking glassThe frequency of the sound is equal to thenatural frequency of the glass. Resonanceoccurs and the glass vibrates with largest
am litude and it breaks.• Tacoma Bridge collapsed in 1940
The wind have the same frequency as thenatural frequency of the vibration of the
bridge. Resonance occurred and caused thebridge to collapse.
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Question 1
A system vibrates with a natural
frequency. If a forced vibration of the
same frequency is given to the system,
A. Damping
B. Vibration
C. Resonance
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• When does an oscillating system
experiences damping?
A. When the amplitude of oscillation
Question 2
B when the period decreases
C when the frequency increases
D when the amplitude remainsconstant