6.1 waves 4 damping & resonance
TRANSCRIPT
6.1WAVESWAVES
DAMPING AND RESONANCE
IN AN OSCILLATING SYSTEMS
LEARNING OUTCOMES
By the end of this lesson, the student shouldbe 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 theswing?
• What can you do to maintain or increase theamplitude?
• 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.
• As time passes, more energy of the pendulum• As time passes, more energy of the pendulumis 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 ofan oscillating system when its energy is lostas heat energy.
• The amplitude of an oscillating system will• The amplitude of an oscillating system willgradually decrease and become zero whenthe oscillation stops.
Activity 2:Damping effect in a
vibrating system• Set up the apparatus as
shown below.• Fix a saw blade
horizontally on the leg ofa laboratory bench usinga laboratory bench usinga G-clamp.
• At the other end of theblade, fix a piece ofplasticine (50 g)
• Move the plasticine tothe position X andrelease it.
• Observe the position Yreached by the plasticine.
• Observe subsequent Ypositions of the plasticine.
Observation:What changes do youobserve in the positionY over severaloscillations?
Conclusion:
Position Y decreases
Conclusion:The position Y becomes _________ to theequilibrium position each time the plasticineoscillates.
closer
• Why does the spring oscillate closer and closer to theequilibrium position?
• Its amplitude decreases.
• What happens to the energy possessed by theplasticine and the blade?
• Energy possess decreases and lost to heat.
• What happens to the oscillation of the blade after a• What happens 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 herbaby to sleep in asarong cradle. Sheneeds tocontinuously movethe sarong cradle upand down.
What happen to the sarong cradlewhen the mother stops moving thecradle up and down?
• Amplitude decreases
Suggest a reason for themovement of the sarong cradlediscussed in (a).
• Damping, lost energy to heat• Damping, lost energy to heat
What must you do if you want tohelp the mother to maintain themovement of the cradle?
• Move the cradle up and down/ give external force
Forced oscillation & Damping
• The swing will stop • The boy is given a• The swing will stopswinging due todamping.
• The boy is given aperiodic push at thecorrect timing willcontinue to swingwithout stopping.
• A system oscillates atits natural frequencywhen no externalforce is applied to it.
• When a periodic force
• If a periodic force isapplied to the swingat its naturalfrequency, theamplitude of the
Forced oscillation & Damping
• When a periodic forceis applied to anoscillating system, theresponse depends onthe frequency of theperiodic force.
amplitude of theswing increases as theenergy of the swingincreases.
• The swing is said tobe in resonance and itoscillates with amaximum amplitude.
Resonance in an oscillating system
• To enable an oscillating system to go oncontinuously, an external force must beapplied to the system.
• The external force supplies energy to the• The external force supplies energy to thesystem. 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 system
Resonance occurs when a system is made tooscillate at a frequency equivalent to itsnatural frequency by an external force.natural frequency by an external force.
The resonating system oscillates at itsmaximum amplitude.
Barton’s pendulum
• The frequency of a simple pendulum dependson the length of the pendulum.
• In Barton’s pendulum experiment, there aremany pendulums tied to the rope. Two ofthe pendulum are of the same lengththe pendulum are of the same length
• When pendulum B oscillates, all the otherpendulums are forced tooscillate.
Barton’s pendulum
• But pendulum D oscillates with the largestamplitude, ie, pendulum D resonates
• Pendulum B and pendulum D are of the samelength.
• Frequency B equal Frequency D• Frequency B equal Frequency D
• Therefore, pendulum B causes pendulum Dto oscillate at its natural frequency.
Effects of resonance
• Breaking glass
The frequency of the sound is equal to thenatural frequency of the glass. Resonanceoccurs and the glass vibrates with largestamplitude and it breaks.amplitude and it breaks.
• Tacoma Bridge collapsed in 1940
The wind have the same frequency as thenatural frequency of the vibration of thebridge. Resonance occurred and caused thebridge to collapse.
Question 1
A system vibrates with a naturalfrequency. If a forced vibration of thesame frequency is given to the system,what will happen?what will happen?
A. Damping
B. Vibration
C. Resonance
• When does an oscillating systemexperiences damping?
A. When the amplitude of oscillationdecreases
Question 2
decreases
B when the period decreases
C when the frequency increases
D when the amplitude remainsconstant