Waves Damping

7/27/2019 Waves Damping

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6.1

WAVEDAMPING AND RESONANCE

IN AN OSCILLATING SYSTEMS



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



• What can you say about the amplitude of the

swing?

• What can you do to maintain or increase the

amplitude?



• At the start, the pendulum oscillateswith maximum amplitude.

• The amplitude of the oscillationdecreases with time and finally stops.



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.



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.



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.



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



• 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.



• 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



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.



• 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.



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.



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.



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.



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.



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.



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



• 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

Waves Damping

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